OF ARTS AND SCIENCES. 91 



optical effect thus produced is like that obtained by superposing ortho- 

 rhombic plates in a definite way. We further know, that, so long as 

 these plates are kept of equal thickness, and their relative position 

 maintained, the chariacter of the effect is independent of their magni- 

 tude. Were the plates indefinitely thin and indefinitely small, there 

 can be no question that a proportional effect would result, which, if 

 indefinitely multiplied by passing the light through a great number of 

 such superposed bundles, must give the same total effect as that ob- 

 tained from a single bundle of thicker and larger plates. Now, mole- 

 cules, in the sense in which we have used the term in connection with 

 the theory we are discussing, are simply the elements of a certain defi- 

 nite orthorhombic structure, and have their analogues in small ortho- 

 rhombic plates, — of mica, for example. By grouping these plates in a 

 definite way, a certain optical effect is produced, without any change of 

 external form. Precisely the same effect is the result of a change 

 caused by heat in the mass of another orthorhombic material having 

 the same crystalline form as the mica plates. Moreover, this change 

 takes place at the exact temperature at which the parts of our protean 

 material must acquire freedom of motion ; and the obvious conclusion 

 is, that in this change the elements of the orthorhombic crystal group 

 themselves anew in the same way in which we group the mica ^ilates 

 in order to obtain a similar result. This is the outline of our argu- 

 ment. To enforce it, we have accessory facts, which show that the 

 theory is in harmony with the accepted principles of polar forces and 

 molecular mechanics, but these it is unnecessary to recapitulate. Of 

 course, demonstration in such a case is out of the question ; but we 

 hope that we have been able to make clear that the theory of mole- 

 cular macling, when stripped of the accessories which the conventional 

 term " molecule " implies, is a close induction from the observed facts. 

 We have only one further point to make, before concluding the dis- 

 cussion of this portion of our subject. The color of the i-hombic 

 iodide of antimony is, as we have said, greenish yellow, recalling that 

 of uranium glass ; and, when either the yellow or the red iodide is dis- 

 solved in disulphide of carbon, the resulting solution has always the 

 greenish yellow color of the first. In this solution, according to all 

 theories, the molecules must be isolated ; and the fact that this charac- 

 teristic color is retained by the rhombic structure, and wholly changed 

 in the hexagonal, is additional evidence that, while in the first the 

 molecules act on the light as units, in the second their individual 

 action must be modified by conditions depending on a peculiar mode 

 of association. 



