OCTOBEE 4, 1901.] 



SCIENCE. 



519 



ject is discussed with his usual suggestive- 

 ness by Clerk Maxwell in the article ' Atom ' 

 in the ' Eucyclopsedia Britannica ' in the 

 volume published in 1875, and he places 

 before the physiologist a curious dilemma. 

 After referring to estimates of the diameter 

 of a molecule made by Loschmidt in 1865, 

 by Stoney in 1868, and by Lord Kelvin 

 (then Sir W. Thomson) in 1870, Clerk 

 Maxwell writes : 



" The diameter and the mass of a mole- 

 cule, as estimated by these methods, are, 

 of course, very small, but by no means in- 

 finitely so. About two millions of the mole- 

 cules of hydrogen in a row would occupy a 

 millimeter, and about two hundred million 

 million million of them would weigh a mil- 

 ligram. These numbers must be con- 

 sidered as exceedingly rough guesses ; they 

 must be corrected by more extensive and 

 accurate experiments as science advances ; 

 but the main result, which appears to be 

 well established, is that the determination 

 of the mass of a molecule is a legitimate 

 object of scientific research, and that this 

 mass is by no means immeasurably small. 



" Loschmidt illustrates these molecular 

 measurements by a comparison with the 

 smallest magnitudes visible by means of a 

 microscope. JSTobert, he tells us, can draw 

 4,000 lines in the breadth of a millimeter. 

 The intervals between these lines can be 

 observed with a good microscope. A cube, 

 whose side is the 4,000th of a millimeter, 

 may be taken as the minimum visible for the 

 observers of the present day. Such a cube 

 would contain from 60 to 100 million 

 molecules of oxygen or of nitrogen ; but 

 since the molecules of organized substances 

 contain on an average about fifty of the 

 more elementary atoms, we may assume 

 that the smallest organized particle vis- 

 ible under the microscope contains about 

 two million molecules of organic matter. 

 At least half of every living organism con- 

 sists of water, so that the smallest living 



being visible under the microscope does not 

 contain more than about a million organic 

 molecules. Some exceedingly simple or- 

 ganism may be supposed built up of not 

 more than a million similar molecules. It 

 is impossible, however, to conceive so small 

 a number suflBcient to form a being fur- 

 nished with a whole system of specialized 

 organs. 



" Thus molecular science sets us face to 

 face with physiological theories. It forbids 

 the physiologist from imagining that struc- 

 tural details of infinitely small dimensions 

 can furnish an explanation of the infinite 

 variety which exists in the properties and 

 functions of the most minute organisms. 



" A microscopic germ is, we know, capable 

 of development into a highly organized ani- 

 mal. Another germ, equally microscopic, 

 becomes when developed an animal of a 

 totally diff'erent kind. Do all the differ- 

 ences, infinite in number, which distinguish 

 the one animal from the other arise each 

 from some difference in the structure of the 

 respective germs ? Even if we admit this 

 as possible, we shall be called upon by the 

 advocates of pangenesis to admit still 

 greater marvels. For the microscopic 

 germ, according to this theory, is no mere 

 individual but a representative body, con- 

 taining members collected from every rank 

 of the long-drawn ramification of the an- 

 cestral tree, the number of these members 

 being amply sufficient not only to furnish 

 the hereditary characteristics of every 

 organ of the body and every habit of the 

 animal from birth to death, but also to af- 

 ford a stock of latent gemmules to be passed 

 on in an inactive state from germ to germ, 

 till at last the ancestral peculiarity which 

 it represents is revived in some remote de- 

 scendant, 



" Some of the exponents of this theory of 

 heredity have attempted to elude the diffi- 

 culty of placing a whole world of wonders 

 within a body so small and so devoid of 



