“A fluid such as air or water seems to us at first glance to be perfectly homogeneous and continuous; we can put more water or less water into this glass, and the experiment seems to suggest to us that the amount of water contained in it can vary by an infinitely small amount, which is the same as saying that water is “indefinitely divisible”. Similarly, a sphere of glass or of quartz, a crystal of alum, are received by our senses as being perfectly continuous, and particularly when we see this alum crystal growing in a supersaturated solution, each of the planes bounding the crystal moves parallel to itself in a continuous manner.”
“However, this can be taken for granted only up to the degree of subtlety reached by the resolving power of our senses which, for example, would certainly be unable to distinguish between two positions of the crystal face one millionth of a millimetre apart. Beyond the things which our senses separate in this manner, our imagination remains free, and ever since ancient times, just as the philosophers who started from the “ full” or the “void”, has hesitated between two hypotheses.”
“For the former, matter remains continuous: “full”, not only (as is reasonable and probable) a little beyond this domain on our scale where our senses make it appear as such, but indefinitely.”
“For the latter, who were the first atomists, all matter consists of minute grains separated by empty gaps; not any hypothesis has been formulated for the structure of these grains themselves, atoms, which were considered as indestructible constituent elements of the Universe.”
“Lastly, and doubtless always, but particularly at the end of the last century, certain scholars considered that since the appearances on our scale were finally the only important ones for us, there was no point in seeking what might exist in an inaccessible domain. I find it very difficult to understand this point of view since what is inaccessible today may becomes accessible tomorrow (as has happened by the invention of the microscope), and also because coherent assumptions on what is still invisible may increase our understanding of the visible.”
“The fundamental laws of chemistry which are well known to you and which are laws of discontinuity (discontinuity between chemical species, and discontinuous variation according to the “ multiple proportions” in the composition of species made from the same simple bodies) then become immediately clear: they are imposed solely by the condition that the molecule constituting a compound contains a necessarily whole number of atoms of each of the simple bodies combined in this compound.”
“And I do not need to tell you that if one admits that “ analogous “ bodies (alkali halides, for example) must have analogous formulae, simple chemical analysis will give for the elements of the same “ family” the ratios of the weights of the atoms, or “atomic weights”, of these elements.”
“In short, in order really to establish the Atomic Theory, it was necessary to obtain the weights and dimensions of the atoms and not only their ratios. A remarkably successful attempt to do this was made about fifty years ago by the physicists who created the kinetic theory of gases by assuming that gases are made of elastic molecules which are on the average fairly widely separated from one another so that, between two collisions, each molecule can move in a straight line, the duration of the collision being negligible in relation to that of the free path.”