EQUIVALENT. On referring to the table of Elementary Sub stances, the reader will obsem that against every substance a certain number is placed, called its " Equivalent," or sometimes its " atomic weight." For instance ; silver has the equivalent 108, iodine 126, carbon 6, sulphur 16, and so on. The meaning of these numbers we will endeavour to explain.
It is found by experiment that substances combine with one another only in c,ertain definite proportions by weight ; and, reasoning on this fact, it has been supposed that matter is not indefinitely divisible, but that bodies are composed of atoms incapable of being divided into smaller fragments, and therefore called " ultimate atoms," these ultimate atoms being of different weight in different bodies. On this hypothesis, the equivalent of any elementary body is the weight of an ultimate atom of that body, the unit of weight being of course indefinitely small, but the same for all substances ; and the equivalent of a compound body is the total weight of all the single atoms composing the compound atom. If, then, we say that the equivalent of silver is 108, we mean that an ultimate atom of silver weig,hs 108 units of weight ; and since in the same scale the equivalent of hydrogen is 1, it appears that the unit of weight in that scale is the weight of an ultimate atom of hydrogen.
But this explanation of the meaning of the table of equivalents involves a hypothesis which, however reasonable it may appear, may nevertheless be false and is after all but a hypothesis. If then
we reject the explanation given according to this hypothesis, and confine ourselves to the facts of the case, the table of equivalents becomes merely a table of the proportions by weight in which different bodies are found to combine.
In order to show the utility of this table, take the following examples :— 1st. In an ounce (480 grains) of chloride of silver, how many grains of silver how many of chlorine are there ? The equivalent of silver is 108, of chlorine 36; the equivalent of chloride of silver is therefore 144. If, then, we multiply 480 by 108, and divide by 144, we get the number of grains of silver in the ounce of chloride, viz. 360; and if we multiply 480 by 36, and divide by 144, we get the number of grains of chlorine, viz. 120. There are, therefore, in an ounce of chloride of silver 360 grains of silver, and 120 grains of chlorine.
• 2nd. Required to know how much iodide of potassium will exactly decompose one ounce (480 grains) of nitrate of silver, neither of these salts containing any water of crystallization.
Nitric acid is composed of 1 part nitmgen and 5 parts oxygen by have nitrous ether, oxalic ether, hydrochloric ether, acetic ether, and so on. As none of these ethers are at present of any use in photography, it would be out of place to describe them in this work.
A table, indicating the quantity of alcohol contained in ether of variable specific gravity, will be found among the tables at the end.