68 



REPORT 1869. 



cal rays that these reactions arc produced. And here I wish to notice, in 

 passing, an interesting fact, important espcciall}' for photographers — I mean 

 the power of intercepting only the chemical rays of solar light, without 

 stopping the luminous rays, j)ossessed by a solution of hisulphate of quinine, 

 well filtered and confined, by means of gutta percha, between two glass 

 plates. This screen, 5 millimetres in thickness, is of an extreme trans- 

 parency to light ; but the chemical rays are intercepted. It might be advan- 

 tageously used instead of the yellow glass, which, in photographic opera- 

 tions, casts on all objects such peculiar hues as to require a special education 

 of the eye which has to judge of the reactions. This screen proved inestimable 

 to me, as it allowed of my disposing and regulating the apparatus suitably 

 beforehand, while it enabled me to permit the chemical rays, which the 

 screen had intercepted, to act at the proper moment only. 



After the experiments on ]S'0'2(80^) (0=8), I tried to unite the most 

 resistant bodies. I introduced into the tube hydrogen and nitrogen perfectly 

 pure and dry, and my surprise was extreme when I beheld the formation of 

 a white cloiid. This iinexpected result, and one which I had considered as 

 utterly impossible, obliged me to look still more closely into the matter, and 

 to proceed with still greater caution. 



The brass rings at the extremities of the tube were luted with the usual 

 resinous cement. Against this cement my first scriiples were now directed ; it 

 might still contain some volatile essence (spirits of turpentine for instance) 

 which might have penetrated into the tube when a good vacuum was formed ; 

 and so small a quantity of matter sufficed perhaps to produce an appreciable 

 result ; this might have given rise to the unexpected cloud. It became 

 necessary to suppress these rings and simplify the apparatus, which I efiected 

 in the following manner. 



I took a cylindrical glass tube (a glass with a foot, Fr. eprouvette) 29 to 30 

 centimetres long and 8 to 9 ui diameter, and 1200 to 2000 cubic centims. 

 in capacity. The upper edge, somewhat bell-mouthed and carefully ground, 

 of above 8 raillims. thickness, was slightly lubricated with a little tallow, wax, 

 and oil melted together. A flat square of plate glass, very transparent, was 

 placed on the oil and sharply pressed down upon it, and some wax was melted 

 all round the contact surface with a hot iron. At the bottom of the cylinder, on 

 the side, a glass stop-cock was carefiiUy fitted in. ^Yith this apparatus the 

 vacuum may be preserved during a long time, even for months together. The 



vacuum is produced, and the gases are introduced l)y the stopcock solely. But 

 there is another method, very important to notice, for introducing into the 

 tube the body to be examined, when it is solid or liquid and volatile. A very 



