different circumstances. A substance passing from one allotropic state to another will often change all its properties, and, to all appearance, become an entirely different body. We have an excellent illustration of this statement in the two states of phosphorus. Red phosphorus and ordinary phosphorus are so unlike that we should consider them distinct substances, if we were not able to prove their identity by converting one into the other. We account for the difference between these two forms by supposing the molecule of the red to be twice as great as that of the ordinary variety. We have instances of the same body appearing in different modifications in sulphur, carbon, and silicon. In view of these facts, chemists ask themselves if, with our present means, we can show in one case that two apparently distinct bodies are but modifications of one and the same substance, shall we not with more extended facilities be able to prove the same of other bodies? For example, there are the four halogens, — fluorine, chlorine, bromine, and iodine; they closely resemble each other in their manner of combination. One thing connected with them may be worthy of notice, and that is, the relation which perhaps exists between their equivalent numbers and their physical condition. Fluorine is a gas, equivalent 19; chlorine a vapor, easily liquefied, equivalent 35.5; bromine a liquid, equivalent 80; iodine a solid, equivalent 127: the ratio between these numbers is pretty nearly as 1, 2, 4, 6. The idea at once occurs to us that these four are but one substance in different molecular conditions; the molecule of iodine being six times condensed, that is, having six times as many atoms as the molecule of fluorine; that of chlorine twice condensed, and that of bromine four times. It is evident that we have by this hypothesis a reason why these elements present a regular gradation from a solid to a gas.
"Again, as an illustration of the importance of a knowledge of the grouping of the atoms in a molecule of any substance, let us observe that we are acquainted with many instances where two or more bodies composed of the same number of atoms and not distinguishable from each other by analysis are yet entirely distinct. It is only from the study of the molecular and atomic constitution of bodies that we will ever attain the transmutation of the metals.
"Very many laborers are even now working in this field. This
