SELECTION UF MATERIAL, APPARATUS, ETC. 



II 



.in fastening together the quartz and glass parts of the cell described in the 

 last section. These parts were first fastened together with Khotinsky 

 cement in the usual way, that is, by heating them in an air bath, to any 

 convenient temperature above the melting point of this cement, and by 

 heating a stick of the adhesive mixture in a Bunsen flame and then applying 

 it to the surfaces of the hot quartz and glass. 



Since this resinous cement is soluble in ethyl and amyl alcohol and other 

 solvents, and because it is attacked by various liquids, such as an aqueous 

 solution of potassium permanganate, it was necessary to coat the exposed sur- 

 faces of the cement with something which was chemically inert towards the 

 solutions to be studied. Such a substance is a solution of rubber in carbon 

 bisulphide. This solution was made and used as follows: From an adequate 

 length of black, soft, rubber connecting-tubing segments about 2 cm. long 

 were cut and heated in an evaporating dish over a Bunsen flame until the 

 sections fused, ran together, and formed a very sticky, viscous liquid. (A 

 single long piece of tubing does not liquefy at all satisfactorily.) The liquid 

 state persisted after the contents of the evaporating dish had been allowed 

 to cool down to about room temperature. Carbon bisulphide was next 

 poured into the dish and the contents of the latter were stirred until a 

 homogeneous solution resulted. 



The relative proportions of the carbon bisulphide and rubber used were 

 immaterial and were determined by convenience only The solution can be 

 retained indefinitely in a tightly stoppered bottle and used whenever needed. 

 A thin layer of the solution was painted over the Khotinsky cement, after 

 which the quartz-glass system was heated in an air bath at about 100 C. 

 until the layer became dry and hard, and was no longer sticky. (Of course, 

 during the first part of the process the transparent elements of the cell had 

 to fit over a suitable wooden "form," because Khotinsky cement softens too 

 much at 100 C. to maintain objects in their proper relative positions.) After 

 this another thin coat of the rubber solution was applied and the heating 

 continued. This succession of operations was repeated until a thick, hard, 

 dark-brown covering for the joints was obtained. It then made little differ- 

 ence whether the original cement were present or not, as the hard rubber 

 held the quartz and glass together very satisfactorily. 



A cement which dissolves readily in water and in acetic acid but which 

 is not affected by ethyl alcohol, amyl alcohol, carbon bisulphide, glycerin, 

 chloroform, ether, benzol, nitrobenzol, aniline oil B, benzaldehyde, toluol, 

 etc., is made by dissolving 2 pounds of pure gelatin in one quart of water and 

 adding to the resulting solution 7 ounces of nitric acid (sp. gr. 1.35 to 1.42). 

 The final solution is colorless and when applied in thin layers dries in a day 

 or so. It is called Dumoulin's liquid glue. This glue does not keep well. 



