PROCEEDINGS OF SOCIETIES. 139 
erystals. Some little allowance must also be made for the dif- 
ferent hygrometric states of the atmosphere. Thus, from two 
salts a series of objects may be produced which, when viewed by 
polarized light, can scarcely be surpassed for beauty. 
The crystals of sulphate of copper and sulphate of magnesia 
will give even more beautiful results than those of the double salt 
in question, and, as far as my observations have gone. at much 
lower temperatures. It is possible that a knowledge of the variety 
of forms which can be thus artificially produced by heat may, 
when applied by scientific men, lead to a more intimate acquaint- 
ance with the ultimate arrangement of atoms in natwral forma- 
tions. 
Mr. Robertson exhibited some Acari obtained from the abdomen 
and chest of the common fowl. The fowl, he stated, appeared 
perfectly healthy, and was killed for dissection. The Acari were 
seen, after the abdomen had been opened, in the form of numerous 
small white specks scattered all over the surface of the viscera, 
and were easily removed with the point of a needle from the 
surface of the peritoneum. They were also present in the chest 
around the bifurcation of the trachea, and dotted over the surface 
of the lung. None were observed under the skin, in the muscular 
tissue, or in the trachea. Examined microscopically, all the Acari 
had four pairs of legs, each composed of six joints. In some the 
