Part 5 (1/2)
After Cavendish had discovered (or rather rediscovered) hydrogen, and had established the fact that this air is extremely inflammable, most chemists began to regard this gas as pure or nearly pure phlogiston, or, at least, as a substance very highly charged with phlogiston. ”Now,” said Priestley, ”when a metal burns phlogiston rushes out of it; if I restore this phlogiston to the metallic calx, I shall convert it back into the metal.”
He then showed by experiment that when calx of iron is heated with hydrogen, the hydrogen disappears and the metal iron is produced.
He seemed, therefore, to have a large experimental basis for his answer to the question, ”What happens when a substance burns?” But at a later time it was proved that iron was also produced by heating the calx of iron with carbon. The antiphlogistic chemists regarded fixed air as composed of carbon and dephlogisticated air; the phlogisteans said it was a substance highly charged with phlogiston. The antiphlogistic school said that calx of iron is composed of iron and dephlogisticated air; the phlogisteans said it was iron deprived of its phlogiston. Here was surely an opportunity for a crucial experiment: when calx of iron is heated with carbon, and iron is produced, there must either be a production of fixed air (which is a non-inflammable gas, and forms a white solid substance when brought into contact with limewater), or there must be an outrush of phlogiston from the carbon. The experiment was tried: a gas was produced which had no action on limewater and which was very inflammable; what could this be but phlogiston, already recognized by this very property of extreme inflammability? Thus the phlogisteans appeared to triumph. But if we examine these experiments made by Priestley with the light thrown on them by subsequent research, we find that they bear the interpretation which he put on them only because they were not accurate; thus, two gases are inflammable, but it by no means follows that these gases are one and the same. We must have more accurate knowledge of the properties of these gases.
The air around a burning body, such as iron, after a time loses the power of supporting combustion; but this is merely a qualitative fact. Accurately to trace the change in the properties of this air, it is absolutely necessary that exact measurements should be made; when this is done, we find that the volume of air diminishes during the combustion, that the burning body gains weight, and that this gain in weight is just equal to the loss in weight undergone by the air. When the inflammable gas produced by heating calx of iron with carbon was carefully and _quant.i.tatively_ a.n.a.lyzed, it was found to consist of carbon and oxygen (dephlogisticated air), but to contain these substances in a proportion different from that in which they existed in fixed air. It was a new kind of air or gas; it was _not_ hydrogen.
This account of Priestley's experiments and conclusions regarding combustion shows how easy it is in natural science to interpret experimental results, especially when these results are not very accurate, in accordance with a favourite theory; and it also ill.u.s.trates one of the lessons so emphatically taught by all scientific study, viz. the necessity of suspending one's judgment until accurate measurements have been made, and the great wisdom of then judging cautiously.
About 1779 Priestley left Lord Shelburne, and went as minister of a chapel to Birmingham, where he remained until 1791.
During his stay in Birmingham, Priestley had a considerable amount of pecuniary help from his friends. He had from Lord Shelburne, according to an agreement made when he entered his service, an annuity of 150 a year for life; some of his friends raised a sum of money annually for him, in order that he might be able to prosecute his researches without the necessity of taking pupils. During the ten years or so after he settled in Birmingham, Priestley did a great deal of chemical work, and made many discoveries, almost entirely in the field of pneumatic chemistry.
Besides the discovery of dephlogisticated air (or oxygen) which has been already described, Priestley discovered and gave some account of the properties of _nitrous air_ (nitric acid), _vitriolic acid air_ (sulphur dioxide), _muriatic acid air_ (hydrochloric acid), and _alkaline air_ (ammonia), etc.
In the course of his researches on the last-named air he showed, that when a succession of electric sparks is pa.s.sed through this gas a great increase in the volume of the gas occurs. This fact was further examined at a later time by Berthollet, who, by measuring the increase in volume undergone by a measured quant.i.ty of ammonia gas, and determining the nature of the gases produced by the pa.s.sage of the electric sparks, proved that ammonia is a compound of hydrogen and nitrogen, and that three volumes of the former gas combine with one volume of the latter to produce two volumes of ammonia gas.
Priestley's experiments on ”inflammable air”--or hydrogen--are important and interesting. The existence of this substance as a definite kind of air had been proved by the accurate researches of Cavendish in 1766. Priestley drew attention to many actions in which this inflammable air is produced, chiefly to those which take place between acids and metals. He showed that inflammable air is not decomposed by electric sparks; but he thought that it was decomposed by long-continued heating in closed tubes made of lead-gla.s.s. Priestley regarded inflammable air as an air containing much phlogiston. He found that tubes of lead-gla.s.s, filled with this air, were blackened when strongly heated for a long time, and he explained this by saying that the lead in the gla.s.s had a great affinity for phlogiston, and drew it out of the inflammable air.
When inflammable air burns in a closed vessel containing common air, the latter after a time loses its property of supporting combustion. Priestley gave what appeared to be a fairly good explanation of this fact, when he said that the inflammable air parted with phlogiston, which, becoming mixed with the ordinary air in the vessel, rendered it unable to support the burning of a candle. He gave a few measurements in support of this explanation; but we now know that the method of a.n.a.lysis which he employed was quite untrustworthy.
Thinking that by measuring the extent to which the _phlogistication_ (we would now say the _deoxidation_) of common air was carried by mixing measured quant.i.ties of common and inflammable airs and exploding this mixture, he might be able to determine the amount of phlogiston in a given volume of inflammable air, he mixed the two airs in gla.s.s tubes, through the sides of which he had cemented two pieces of wire, sealed the tubes, and exploded the mixture by pa.s.sing electric sparks from wire to wire. The residual air now contained, according to Priestley, more phlogiston, and therefore relatively less dephlogisticated air than before the explosion.
He made various measurements of the quant.i.ties of dephlogisticated air in the tubes, but without getting any constant results. He noticed that after the explosions the insides of the tubes were covered with moisture. At a later time he exploded a mixture of dephlogisticated and inflammable airs (oxygen and hydrogen) in a copper globe, and recorded the fact that after the explosion the globe contained a little water. Priestley was here apparently on the eve of a great discovery. ”In looking for one thing,”
says Priestley, ”I have generally found another, and sometimes a thing of much more value than that which I was in quest of.” Had he performed the experiment of exploding dephlogisticated and inflammable airs with more care, and had he made sure that the airs used were quite dry before the explosion, he would probably have found a thing of indeed much more value than that of which he was in quest; he would probably have discovered the compound nature of water--a discovery which was made by Cavendish three or four years after these experiments described by Priestley.
Some very curious observations were made by Priestley regarding the colour of the gas obtained by heating ”spirit of nitre” (_i.e._ nitric acid). He showed that a yellow gas or air is obtained by heating colourless liquid spirit of nitre in a sealed gla.s.s tube, and that as the heating is continued the colour of the gas gets darker, until it is finally very dark orange red. These experiments have found an explanation only in quite recent times.
Another discovery made by Priestley while in Birmingham, viz. that an acid is formed when electric sparks are pa.s.sed through ordinary air for some time, led, in the hands of Cavendish--an experimenter who was as careful and deliberate as Priestley was rapid and careless--to the demonstration of the composition of nitric acid.
Many observations were made by Priestley on the effects of various airs on growing plants and living animals; indeed, one of his customary methods of testing different airs was to put a mouse into each and watch the effects of the air on its breathing. He grew sprigs of mint in common air, in dephlogisticated air (oxygen), and in phlogisticated air (nitrogen, but probably not pure); the sprig in the last-named air grew best, while that in the dephlogisticated air soon appeared sickly. He also showed that air which has been rendered ”noxious” by the burning of a candle in it, or by respiration or putrefaction, could be restored to its original state by the action of growing plants. He thought that the air was in the first instance rendered noxious by being impregnated with phlogiston, and that the plant restored the air by removing this phlogiston. Thus Priestley distinctly showed that (to use his own words) ”it is very probable that the injury which is continually done to the atmosphere by the respiration of such a number of animals as breathe it, and the putrefaction of such vast ma.s.ses, both of vegetable and animal substances, exposed to it, is, in part at least, repaired by the vegetable creation.” But from want of quant.i.tative experiments he failed to give any just explanation of the process whereby this ”reparation” is accomplished.
During his stay in Birmingham, Priestley was busily engaged, as was his wont during life, in writing metaphysical and theological treatises and pamphlets.
At this time the minds of men in England were much excited by the events of the French Revolution, then being enacted before them. Priestley and some of his friends were known to sympathize with the French people in this great struggle, as they had been on the side of the Americans in the War of Independence. Priestley's political opinions had, in fact, always been more advanced than the average opinion of his age; by some he was regarded as a dangerous character. But if we read what he lays down as a fundamental proposition in the ”Essay on the First Principles of Civil Government”
(1768), we cannot surely find anything very startling.
”It must be understood, whether it be expressed or not, that all people live in society for their mutual advantage; so that the good and happiness of the members, that is the majority of the members of any state, is the great standard by which everything relating to that state must be finally determined. And though it may be supposed that a body of people may be bound by a voluntary resignation of all their rights to a single person, or to a few, it can never be supposed that the resignation is obligatory on their posterity, because it is manifestly contrary to the good of the whole that it should be so.”
Priestley proposed many political reforms, but he was decidedly of opinion that these ought to be brought about gradually. He was in favour of abolis.h.i.+ng all religious State establishments, and was a declared enemy to the Church of England. His controversies with the clergy of Birmingham helped to stir up a section of public opinion against him, and to bring about the condemnation of his writings in many parts of the country; he was also unfortunate in making an enemy of Mr. Burke, who spoke against him and his writings in the House of Commons.
In the year 1791, the day of the anniversary of the taking of the Bastille was celebrated by some of Priestley's friends in Birmingham. On that day a senseless mob, raising the cry of ”Church and King,” caused a riot in the town. Finding that they were not checked by those in authority, they after a time attacked and burned Dr. Priestley's meeting-house, and then destroyed his dwelling-house, and the houses of several other Dissenters in the town. One of his sons barely escaped with his life. He himself found it necessary to leave Birmingham for London, as he considered his life to be in danger. Many of his ma.n.u.scripts, his library, and much of his apparatus were destroyed, and his house was burned.
A congregation at Hackney had the courage at this time to invite Priestley to become their minister. Here he remained for about three years, ministering to the congregation, and pursuing his chemical and other experiments with the help of apparatus and books which had been supplied by his friends, and by the expenditure of part of the sum, too small to cover his losses, given him by Government in consideration of the damage done to his property in the riots at Birmingham.
But finding himself more and more isolated and lonely, especially after the departure of his three sons to America, which occurred during these years, he at last resolved to follow them, and spend the remainder of his days in the New World. Although Priestley had been very badly treated by a considerable section of the English people, yet he left his native country ”without any resentment or ill will.” ”When the time for reflection,” he says, ”shall come, my countrymen will, I am confident, do me more justice.”
He left England in 1795, and settled at Northumberland, in Pennsylvania, about a hundred and thirty miles north-west of Philadelphia. By the help of his friends in England he was enabled to build a house and establish a laboratory and a library; an income was also secured sufficient to maintain him in moderate comfort.
The chair of chemistry in the University of Philadelphia was offered to him, and he was also invited to the charge of a Unitarian chapel in New York; but he preferred to remain quietly at work in his laboratory and library, rather than again to enter into the noisy battle of life. In America he published several writings. Of his chemical discoveries made after leaving England, the most important was that an inflammable gas is obtained by heating metallic calces with carbon. The production of this gas was regarded by Priestley as an indisputable proof of the justness of the theory of phlogiston (see pp. 63, 64).