tomical elements from which direct analysis extracts a certain number of chemical principles. Here the anatomist's work ends. The chemist steps in, and recognizes in these principles definite kinds arising from the combination, in fixed and determinate proportions, of a certain number of principles that cannot be decomposed, substantially indestructible, to which he gives the name of simple bodies. Carbon, nitrogen, oxygen, hydrogen, sulphur, phosphorus, calcium, iron, which thus set a limit to experimental analysis of the most complex bodies, are simple substances, that is to say, they are the original and irresolvable radicals of the tissue of things.
We now know that matter is not indefinitely divisible, and that the smallest parts of the various simple substances existing in those that are naturally compound have not all the same dimensions, nor equal weights. Chemistry, by a course of analyses and measurements, has succeeded in determining the weights of atoms of the different elements, that is to say, taking as a unit an atom of the lightest element, hydrogen, in determining the weight of the atoms which are equivalent to this conventional unit in the various combinations. Though many savants continue to maintain that atomic weights are nothing but relations, and that the existence of atoms is a mere logical device, it seems more reasonable to admit, with the majority of those who have studied this difficult problem closely, that these atoms are actual realities, while it may be very far from easy to settle precisely their absolute dimensions. In any case, we may affirm that these dimensions are very much less than those presented by the particles of matter subjected to the most powerful and subtle methods of division, or decomposed by the imagination into its minutest elements. "Let man," says Pascal, "investigate the smallest things of all he knows; let this dot of an insect, for instance, exhibit to him in its diminutive body parts incomparably more diminutive, jointed limbs, veins in those limbs, blood in those veins, in that blood humors, and drops within those humors—let him, still subdividing these finest points, exhaust his power of conception, and let the minutest object his fancy can shape be that one of which we are now speaking—he may, perhaps, suppose that to be the extreme of minuteness in Nature. I will make him discover yet a new abyss within it. I will draw for him not merely the visible universe, but all besides that his imagination can grasp, the immensity of Nature, within the confines of that imperceptible atom." In this Pascal displays a feeling as true as it is deep of the infinitely small, and it is interesting to observe how the amazing revelations of the microscopic world have justified his eloquence and foresight; and yet this microscopic world, whose minutest representatives, such as vibrios and bacteria, are hardly less than the ten-thousandth part of 1⁄25 of an inch, how coarse it is compared with the particles thrown off by odorous bodies, and with the inconceivably minute quantities which chemistry, physics, and
