SCIENCE IX THE LAST HALF CENTURY. 69 



considerations upon the development of all three. The peculiar merit 

 of our epoch is that it has shown how these hypotheses connect a vast 

 number of seemingly independent partial generalizations; that it has 

 given them that precision of expression which is necessary for their 

 exact verification ; and that it has practically proved their value as 

 guides to the discovery of new truth. All three doctrines are intimately 

 connected, and each is applicable to the whole physical cosmos. But, 

 as might have been expected from the nature of the case, the first t\^o 

 grew, main ly, out of the consideration of physico-chemical phenomena 

 while the third, in great measure, owes its rehabilitation, if not its origin, 

 to the study of biological phenomena. 



1. STRUCTURE OF MATTER. 



In the early decades of this century, a number of important truths 

 applicable, in part, to matter in general, and, in part, to particular 

 forms of matter, had been ascertained by the physicists and chemists. 



The laws of motion of visible and tangible — or molar matter had been 

 worked out to a great degree of refinement and embodied in the branches 

 of science known as Mechanics, Hydrostatics, and Pneumatics. These 

 laws had been shown to hold good, so far as they could be checked by 

 observation and experiment, throughout the universe, on the assump- 

 tion that all such masses of matter possessed inertia and were suscep- 

 tible of acquiring motion in two ways, firstly by impact, or impulse 

 from without ; and secondly, by the operation of certain hypothetical 

 causes of motion termed " forces," which were usually supposed to be 

 resident in the particles of the masses themselves, and to operate at a 

 distance, in such a way as to tend to draw any two such masses to- 

 gether, or to separate them more widely. 



With respect to the ultimate constitution of these masses, the same 

 two antagonistic opinions which had existed since the time of Democ- 

 ritus and of Aristotle were still face to face. According to the one, 

 matter was discontinuous- and consisted of minute indivisible particles 

 or atoms, separated by a universal vacuum; according to the other, it 

 was continuous, and the finest distinguishable, or imaginable, particles 

 w ere scattered through the attenuated general substance of . he plenum. 

 A rough analogy to the latter case would be afforded by granules of 

 ice ditfused through water; to the former, such granulesdiffut^ed through 

 absolutely empty space. 



In the latter part of the eighteenth century the chemists had ar- 

 rived at several very important generalizations respecting those prop- 

 erties of matter with which they were especially concerned. However 

 plainly ponderable matter seemed to be originated and destroyed in 

 their operations, they proved that as mass or body, it remained iude- 

 structible, and ingenerable; and that so far, it varied only in its per- 

 ceptibility by our senses. The course of investigation further proved 

 that a certain number of the cliemically separable kinds of matter were 



