Intelligence and Miscellaneous Articles. 157 



Y\ r e have it so on the screen. Whether it be right or wrong in 

 regard to other matter, I am convinced that in silver bromide 

 we have the possible existence of two sizes of molecules. In the 

 blue film we have the presence of both ; when only the larger 

 size is present we shall have a compound which is very much 

 more sensitive to the lower end of the spectrum than it is to the 

 upper. Allow me to say that these views are not original, except 

 in so far as they are applied to this subject. "Whether they are 

 correct or not, they have formed a good working hypothesis which 

 has led me to the results obtained. 



Before closing I must refer to the comparative lengths of ex- 

 posure required for these photographs. My impression is that at 

 A the exposure required is about 25 times that required for Gr, in 

 a fairly bright sun at midday ; for the ultra red, as far as I have 

 gone, I should say about 35 times. These are only approximations, 

 but still wall enable you to form some idea of the sensitiveness. I 

 show you a photograph taken about 4 p.m. on March 18. The 

 exposure was about 50 seconds. Tou will see that the red end is 

 as strong as the blue, with the yellow much lacking in density. In 

 other words, the yellow rays are nearly inactive. 



The photograph which I showed you of the four orders of the 

 spectrum was taken on April 3, at 2.30. It had an exposure of 

 one minute and a half. The photograph in which the furthest 

 band of lines was seen had an expos ure of 12 minutes, on the 

 8th, at 2.30. The slit was in this case closed as nearly as pos- 

 sible. 



In conclusion, I have to remark that in a short time I hope to 

 reduce the exposures considerably. In the course of some investi- 

 gations, the results of which have just been communicated to the 

 Royal Society, I found that the red rays could oxidize a photo- 

 graphic image as well as form it, and that in an oxidized state it 

 was unable to be developed. If the tendency of the sensitive 

 compound to become oxidized exceeded its tendency to become 

 reduced, no image could be developed. By exposing in vacuo, or in 

 a nitrogen atmosphere, I hope to eliminate altogether this oxidizing 

 effect, and so get firmer images. — Monthly Notices of the Royal 

 Astronomical Society, April 1878. 



ON THE FRICTION OF VAPOURS. BY DR. J. PULUJ. 



Friction-experiments with vibrating disks confirm for vapours 

 also the law that the friction is independent of the pressure up to 

 the limit of saturation, and the law of its proportionality to the 

 absolute temperature, which latter law has been experimentally 

 proved by A. v. Obermeyer and the author for more easily com- 

 pressible gases. 



For ether-vapour the calculation gave, within the temperature- 



