Answers to Quertes. 5 
loading them with dye has no other effect than to take away their 
natural appearance. GH. B: 
321.—Arranged Diatoms.—In the Lugucrer for March, 1887, I 
gave an account of my method of mounting these, which has 
proved very successful. I think it will be found satisfactory. The 
cement used is a solution of white shellac in alcohol. G) EB: 
323.—Barbados Earth.—I believe the method described in 
* Davies on Mounting,” is still very generally used. To 
disintegrate the earth he gives the following plan :—‘ Take 3 oz. 
of the earth (lumps are best), and break into rather small 
fragments, put 3 or 4 ozs. of common washing soda into a tin 
saucepan holding one pint, and half fill it with water. Boil 
strongly, and having thrown in the earth boil it for half-an-hour. 
Pour nine-tenths of this into a large glass vessel, containing 3 or 4 
quarts of water, and gently crush the remaining lumps with a soft 
brush, add soda and water as before, and boil again, then pour off 
the liquid into the large vessel and repeat until nothing of value 
remains. Stir the large vessel with an ivory spatula, let it stand 
for three minutes, and pour gently off nine-tenths of the contents, 
when the shells will be left partially freed, only like sand.” The 
boiling process may then be continued. GH: B: 
325.—Weight of the Earth.—To measure the earth’s mass, in 
terms of a known unit of mass, such as a pound, it is necessary 
to compare the attraction of gravitation exerted by the earth on a 
given body, with that exerted by a heavy sphere whose mass is 
known in pounds, acting on the same body, when the latter is 
placed at a known distance from its centre. ‘Then, since the 
attractions of spheres vary directly as their masses, and inversely 
as the square of the distance from their centres, itis only necessary 
to know the radius of the earth, in order to complete the determi- 
nation both of its mass and density. 
To measure the attraction between the two known masses is, 
however, a matter of considerable difficulty, owing to its smallness, 
and the best way of doing this is that known as the “ Cavendish 
Experiment,” in which this is done by means of a sort of tortion 
balance. Two equal balls are attached to a light horizontal beam 
suspended by a thin wire, and two heavy spheres are brought near 
the balls in such a way as to turn the rod about the wire, till the 
attraction of the spheres is balanced by the elasticity of the wire. 
The angle through which the beam turns is then measured, and 
the elasticity of the wire is supposed known. By this means the 
attraction of the spheres are determined. There are many 
modifications of this method required in practice, but they are of 
