To render this instrument of more service, a small stem is fixed at the end of the tube, upon which weights, like that at g, may be placed. Suppose then the weight of the instrument is 10 dwts., and by being placed in any kind of spirit it sinks to a certain point L, it will require an additional weight, suppose 1.6 dwt. to cause it to sink to the same depth in water: in this case the specific gravity of the water to the spirit will be as 11.6 to 10. By the addition of different weights the specific gravity of any kind of liquor is easily found. The point L should be so placed as to mark the exact depth to which the instrument will sink in the liquor that has the least specific gravity.

Charles. But you always make the specific gravity of water 1, for the sake of a standard.

Father. Right: and to find the specific gravity of the spirit compared with water at 1, I say, as 11.6:1:: 10.862 nearly, so that I should put the specific gravity of this spirit down at .862 in a table where water was marked 1 and as a cubic foot of water weighs 1000 ounces, a cubic foot of this spirit would weigh 862 ounces, which is generally the standard of pure rectified spirit.

Emma. Is this what is usually called spirits of wine?

Father. No: it is the alcohol of the chemists, one pint of which added to a pint of water make a quart nearly of common spirits of wine.

Charles. You said .862 was generally the specific gravity of alcohol: what causes the difference at other times?

Father. It is not always manu

factured of equal strength: and the same fluids vary in respect to their specific gravity by the different degrees of heat and cold in the atmosphere. The cold of winter condenses the fluid, and increases the specific gravity; the heat of summer causes an expansion of the fluid, and a diminution of its specific gravity.

Emma. You said just now that a pint of water added to a pint of alcohol made nearly a quart of spirits of wine; surely two pints make a full quart?

Father. Indeed they will not. A pint of water added to a pint of water will make a quart: and a pint of spirit added to a pint of spirit will make a quart; but mix a pint of spirit with a pint of water, and there is a certain chemical union or penetration between the particles of the

two fluids, so that they will not make a quart. This subject we shall resume in our Chemical Conversations.*

*See Dialogues in Chemistry, Vol. I. p. 46.

CONVERSATION XVII.

Of the Hydrometer, and Swimming.

CHARLES. To what purposes is the hydrometer applied?

Father. It is used in breweries and distilleries to ascertain the strength of their different liquors and by this instrument the excise officers guage the spirit, and thereby determine the duties to be paid to the

revenue.

I think from the time we have spent in considering the specific gravity of different bodies, you will be at no loss to account for a variety of circumstances that may present them