Part 3 (2/2)

In all cases where hydrofluoric acid is used, or stored, it is of great importance to keep it well away from any optical instruments, as the hly destructive corrosion of any glass surfaces

_Methods of Calibration_--In the case of apparatus for voluh some of the smaller subdivisions are often made by measurement When the subdivisions are made in this way it is of importance to see that the walls of the tube or vessel to be calibrated are parallel Great errors arise in solect of this precaution A convenientfor parallelish in successive quantities of th occupied by each successive quantity will indicate any change in the bore In the case of capillary tubes, it is convenient to introduce an unweighed quantity of th accurately, and then tothe tube or by the application of air pressure A e will indicate whether the bore is approximately parallel or not Neither of these methods is to be relied on without a careful examination of the tube, as it ularities in the bore which compensate for each other, and do not, therefore, affect the voluth

Obviously, the smaller the quantity of reater the number of observations made, the less risk will there be of such an error A liquid, such as water or alcohol, which wets the glass is not suitable for such a test, unless special precautions are taken

When, however, a pipette or burette has to be calibrated to deliver a certain volume of water, the final calibration must be made with this liquid Thus, the burette would first be calibrated by weighing in definite quantities of rammes (1 cc at 15C), each of the 1 cc divisions should be

The burette is now filled with a solution of potassium bi+chromate and sulphuric acid and allowed to soak for some time; the bi+chromate is washed out and distilled water is put in Successive quantities of water are run out of the jet, a fixed tihts of the quantities delivered are noted This procedure will give the necessary data for altering theso that it may correspond to 1 cc _delivered_ Each 1 cc division is now divided into tenths by the s should be made when the subdivision is completed

_Subdivision of Graduations_--Mark out the spaces to be subdivided on a sheet of paper Take a reliable ruler on which any convenient length is divided into the desired nule that the liap No parallel lines through thea Scale_--When a scale has been prepared on paper and it is necessary to copy that scale on the waxed-glass surface for etching, a convenienta sharp needle passing through it at either end The scale and object to be marked are fastened in line with one another, and the caliper bar is used froh a minute portion of a circle, which provided that the bar is two or three feet in length, will not introduce any perceptible error in a scale of say a quarter of an inch in width The arrange 15]

_Graduating a Ther that the ther in which the bore is parallel and free fro point and boiling point The intervening space may then be divided into 100 (if the therrade) or 180 (if Fahrenheit) This division iven under ”Subdivisions of Graduations” A ther, as the glass bulb tends to contract

_Joining Glass and Metal_--It sometimes happens that one needs to lass and metal tube than can be obtained by htly different methods may be elass with platinulass with platinu to redness; a layer of copper is then deposited electrolytically on the treated surface of the glass, and soldering is carried out in the usual manner

McKelvy and Taylor call attention to two other methods in the _Journal of the Chemical Society_ for Septelass is coated with platinu it with a suspension of platinu until the oil is burnt off The metal tube is then tinned on its inner side and soldered to the prepared glass, slightly acid zinc chloride being used as a flux

In the second method, a joint is hts of zinc oxide, borax, and powdered soda-glass fused together This is coated on the inner surface of the lass tube, which has had the end slightly flanged to give support, is inserted Fusion of the flux is co Glass_--In all cases where it is intended to deposit a silveris essential

Prolonged soaking in a hot solution of potassium bi+chromate which has been acidified with sulphuric acid will often prove useful The glass should then be washed thoroughly, rinsed in distilled water, and the solution should then be used

There aresolution, but that used in Martin's rammes Distilled Water 1000 c crammes Distilled Water 1000 c craar Candy 100 grammes Distilled Water 1000 c crammes

Boil for ten minutes, and when cool add:--

Alcohol 200 c cm

Distilled Water to 2000 c cether also equal parts of C and D in another vessel Thenvessel and suspend the glass to be silvered face doards in the solution Or if a vessel has to be silvered on the inside, the solution is poured in In this case, the deposition of silverthe vessel to be silvered in ater

In working with a silver solution containing ammonia or a an explosive silver co such solutions longer than is necessary, and to bear inboth silver and ammonia may have explosive properties, especially when dry

CHAPTER V

Exte Apparatus--The Use of Oil or other Fuels--Making Small Rods and Tubes from Glass Scrap--The Examination of Manufactured Apparatus with the View to Discovering the Methods Used in Manufacture--Su