Chemistry and Physics. 111 
‘Tn this particular, uniaxial differ from biaxial crystals, in: which the 
rays of heat exhibit differences in three directions at right angles to 
each other. 
If the heat is polarized, differences are observed even in the same 
direction according to the position of the plane of polarization. 
Transmissions perpendicular to the axis exhibit however in this case 
corresponding peculiarities. 
is only when the rays are transmitted along the axis that their 
passage and quality are independent of the position of the plane of 
polarization. 
amethyst and idocrase differ, under otherwise similar circumstances, 
diathermanous substances 
All these observations correspond completely with those which the 
author formerly made with rock crystal, bery! and tourmaline. 
III. Even in crystals belonging to the regular system like colored 
of heat produced by the absorption of gases by porous solids, an 
Compared the quantity of heat thus set free with the latent heats of va- 
porization and liquefaction of the gases in question. ‘The author _ 
le gases are absorbable in the following order: ammonia, muriatic 
acid, sulphurous acid, protoxyd of nitrogen, carbonic acid. 
| ses may be classed in the same order relatively to the - 
amount of heat disengaged during the process of absorption to satura- 
gra 
ceeds the latent heat of liquefaction of equal weights of the same gases. 
_ Thus the latent heat of sulphurous acid is 88°3 units, while its heat of 
absorption is 150-1 units: the latent heat of protoxyd of nitrogen is 
100°6 and its heat of absorption 1483. In the case of carbonic acid 
