From experiments made on the toxic action of the chloride, bromide, and iodide of potassium, Bouchardat and Stewart Cooper came to the conclusion that a relation existed between the physiological activity of elements and their atomic weight, the activity being inversely as their atomic weight, e.g. fluorine (atomic weight, 19) being more active than chlorine (atomic weight, 35.5).

In 1867, Rabuteau made a number of experiments from which he concluded that Bouchardat was correct in saying that the physiological activity of the monatomic metalloids was in inverse proportion to their atomic weight, while that of the diatomic metalloids increased directly with their atomic weight: selenium being more active than sulphur.

He considered also that he had discovered a new law regarding the relation between the atomic weight and the physiological activity of metals : viz., that the activity of metals increases with their atomic weight. He afterwards qualified this statement by saying that the poisonous action increased with the atomic weight amongst elements belonging to the same group. Thus potassium (atomic weight, 39) is more poisonous than sodium (23), and barium (137) than calcium (40). But it has been shown by Huse-mann that lithium is much more poisonous than sodium, and his results have been confirmed by Richet.

In the following table the lethal activity of various metals is given as determined by Richet, and of the metals belonging to the groups of the alkalis and earths as determined by Richet, by Cash and myself, and by Botkin ran Where the position of the metals in the tables is different the symbols are printed in italics. The most active, Hg, is first; the least active, Na or Ca, last Richet's experiments were made upon fish, and the substances were added to the water in which the animals were swimming. The experiments of Cash and myself were made upon frogs, and the substances were injected subcutaneously. Botkin's experiments 1 were made upon dogs, and the substances were injected directly into the circulation.

1 See Hartley, Phil. Trans., Part II. 1885.

2 Russell and Lapraik, Journ. Chem. Soc, April 1881.

3 Abney and Festing, Phil. Trans., 1882, p. 887.

Richet

Brnnton and Cash

Botkin, junr.

Atomic Weight

Richet

Bruntou and Cash

Botkiu, junr.

Atomic Weight

Hg

-

-

200

-

Cs

Cs

133

Cu

-

-

63.4

Li

Li

Li

7

Zn

-

-

65

Mn

-

-

55

Fe

-

-

56

Ba

-

-

137

Cd

-

-

111.2

Mg

-

-

24

NH4

-

-

18

-

La

-

139

K

K

K

39

-

Di

-

145.4

-

Be

-

9

-

Er

-

166

-

Rb

Rb

85.3

Sr

Sr

-

87.4

Ni

-

-

58

-

Yt

-

89.8

Co

-

-

58

Ca

Na

-

40

23

-

Ba

-

137

Na

Ca

-

23

40

-

NH4

-

18

It is possible that the differences observed were due to the differences in the animals on which the experiments were made, or in the way of applying the poison. Botkin's table, so far as it goes, agrees perfectly with Cash's and mine, and there is a general correspondence also between Richet's results and ours, although there are several differences in particulars.

It is thus evident that the relationship between atomic weight and physiological action is no simple one. But indeed, on looking into the matter more closely, we could hardly expect it would be. For the toxic action of an element may depend upon its effect on the muscles, nerves, nerve-centres, blood, or on the digestive or excretory systems. These differ from one another in their composition, and while it is possible that the elements belonging to a certain group may have relations varying with their atomic weight to individual organs or structures, we can hardly expect those relationships to be the same to all organs.

Thus an element with one atomic weight may prove fatal, by affecting the muscular power of an animal, while another with an atomic weight either higher or lower, may be still more deadly by affecting the nervous system or heart.

What we want, therefore, is not a general relationship between atomic weight and toxic action, but a knowledge of the particular relationships of each group of elements to each organ and tissue of the body.