226 
NATURAL-PHILOSOPHICAL COLLECTIONS. 
Extract from a letter from M. Kupffer to M. Arago, concerning the composition 
of the Atmosphere ai Kazan. 
THE observations which I have conducted in this place, on the composition of 
the atmosphere, may serve as a supplement to those which have already been 
made in so many different situations, and from which it results, that, independ- 
ently of climates and cultivation, the proportion of the two principal constituents 
of the air is every where the same. In civilized Europe, one might anticipate 
that the slightest difference in this respect would soon be removed by the admixture 
which portions of air, not separated by more than some hundreds of leagues, must 
experience from the influence of the winds; but Kazan, which is surrounded, on 
one side, by a country in a low state of cultivation, and on the other by the steps 
and immense forests of Siberia, where vegetation is lifeless during the greater por- 
tion of the year, might have an atmosphere somewhat different from the rest of 
Europe. The instrument I used was the Eudiomater of Volta; 198 parts of at- 
mospheric air mixed with 99 parts of hydrogen gas, afforded me constantly 171 to 
172 parts, after detonation; which gives 21,0 to 21,2 of oxygen in 100 parts of 
atmospheric air. I have taken the greatest care to experiment always at the same 
temperature, and under the same pressure, so no correction is necessary in this 
respect. ‘The gases were saturated with humidity ; for I made my experiments 
under water. Ann. de Chim. Aug. 1829. 
Extract from a Memoir on the Causes of Diffraction, by M. Haldat. 
The phenomena of diffraction, the examination of which has furnished of late 
such powerful arguments against the hypothesis of Newton, and has drawn our 
philesophers towards the opinion of Descartes, seem to M. Haldat not to have 
been sufficiently discussed in relation to the circumstances which may modify 
them, and to the discovery of the cause. Under this point of view he has tried a 
great number of experiments in which the bodies which produced diffraction, (and 
which he names diffringents), have been submitted to the action of the most pro- 
per agents for modifying them; and as the attractive force is the property on 
which the Newtonians have supposed the diffraction to depend, he has brought 
into action in his experiments all the agents most capable ef influencing it. 
After having assured himself that, as many experimentalists had announced, 
this phenomenon was neither modified by the density nor by the chemical nature 
of the bodies, he turned his attention towards the greatest powers in nature, caloric, 
electricity, magnetism, electro-chemical currents and, lastly, affinity, so powerful 
in altering the attractive force, were successively, as well as at the same time em- 
ployed to modify the state of the bodies while they were exercising over the lu- 
minous rays, the influence by which diffraction is produced, without the phenome- 
na by which it is characterised undergoing any sensible change. Thus metallic 
wires, diffringent plates of iron, of copper, and of silver, have been heated to red- 
ness and cooled again to —10°. without the toloured bands produced by their 
action upon the luminous rays having presented any appreciable difierence from 
those which the same bodies exhibited at the medium temperature of the atmos- 
phere. 
Wires of diffringent plates have been traversed by currents of common eleciri- 
city, by violent discharges of powerful batteries, and by electro-chemical currents 
suificiently energetic to redden and to dissolve them. Moving currents have been 
employed in the same or in opposite directions, a ray of light has been received upon 
the inclined plane of difiringent plates, which had been powerfully magnetised, ° 
without the phenomena undergoing any observable alteration. Rays of light have 
even been traversed by rays of vivid fame, by powerful electrical currents or dis- 
charges, before their arrival at the diffringent plates or wires, without any change 
being manifested in the fringes and other phenomena of attraction. The obscure 
