TRANSACTIONS OF THE SECTIONS. 55 



open to me under sucli circumstances is to dii'ect jowx attention to the ideas wliieli 

 at present guide chemists in their researches, to place in a clear light the objects 

 they are stri^-ing to attain, and to indicate the dii-ection of scientific thought of oui- 

 time. To do this is by no means an easy task ; for the more manifold and diver- 

 sified the objects of a science become, the more numerous and extensive its rela- 

 tions with other branches of knowledge, the more difficult it becomes to di'aw a 

 picture of its actuiil condition. 



It is always an excellent recommendation of a theory or hj-pothesis when 

 amongst the cidtivators of the science to which it pertains very little difierence of 

 opinion exists as regards its admissibility and scientific value. This is in a high 

 degree the case with regard to the atomic theory. The vast majority of chemists, 

 I believe, accept this theory as the most suitable exponent of the fimdamental 

 truths of their science; and certainly if the quality of the tree may be judged by 

 its fruit there is no other view which furnishes a clearer image to om- minds of the 

 chemical constitution of bodies, and at the same time conducts to the discovery of so 

 many important facts and relations. According to Daltou's profound h^'pothesis all 

 bodies are supposed to be composed of atoms of infinitely small dimensions. But 

 these atoms are supposed not to be single ; two or more of them are held together 

 by certain forces and thus constitute what is called a molecule. One atom of carbon, 

 one atom of calcium, and three atoms of oxygen, joined together by the force called 

 chemical affinity, constitute a molecide of carbonate of lime. Vast numbers of 

 such molecules bound to each other by the force of cohesion form a visible piece of 

 chalk. If a chemist wishes to examine a body, his first endeavour is to ascertain 

 of what sort of atoms the body is formed. This is a mere matter of experiment. 

 He next determines how many of such atoms are contained in each molecide of 

 the body, and finally he ascertains how these atoms are arranged, or, more correctly, 

 combined within the molecule ; for it is quite clear that a substance like saltpetre, 

 which contains one atom of nitrogen, one of potassium, and three of oxygen, may 

 have these atoms an-anged in very difterent manners and stiU have the same com- 

 position. We might assume the potassium and nitrogen in more intimate union, 

 nearer to each other than they are to oxygen, or we might consider nitrogen and 

 oxygen more closely packed together, and, so to speak, attached as a whole to the 

 potassium ; in both cases, saltpeti-e would have in each molecule the same number 

 of atoms, and the weight of the molecule would be the same. The three determi- 

 nations just mentioned are of fundamental importance to the chemist; not that 

 such iuqiuries are the only ones which interest him, for we shall in the sequel 

 notice others of almost equal importance. 



Nor must it be supposed that questions of this nature are of quite a modern date ; 

 for Leucippus, .500 B.C., appears to have sought to explain the nature of things by 

 the assumption that they are formed by the union of small particles, which latter re- 

 ceived the name of atoms from Epicm-us. It is true the notion of atoms as conceived 

 by the Grecian philosophers is not quite the same as ours, but their specidations 

 contain our notions pretty much in the same way as the acorn contains the oak tree. 



The determination of the quality of the atoms in a molecule, or the analysis of 

 the latter, has not undergone many changes during the last few j-eiirs, and the same 

 may be said about the fiiiding of the relative weight of a moleciile, or the determi- 

 nation of the number of atoms which are contained in it. With regard to the latter 

 point, however, it may be mentioned that Avogadro's hypothesis, according to 

 which equal volumes of gaseous substances, measured at the same temperatiu-e and 

 pressm'e, contain the same number of moleciUes, grades us chiefly in assigning to 

 each molecule its relative weight and its number of atoms ; this hjTjOthesis has 

 won more and more the confidence of chemists, and it is now admitted to hold 

 good in nearly all well-examined cases. 



Om" ■\dews relative to the combinations of atoms in molecules, and our methods 

 of ascertaining this arrangement, have, however, undergone gTeat alterations and 

 received great additions during the last ten or fifteen years. To a consideration of 

 these changes I vnll now, for a short time, invite yom* attention. Since our modern 

 views, however, originated in a great measure from the study of organic bodies, and 

 since the majority of chemists now devote their time and labour thereto, I shall 

 confine my remarks principally to the organic branch of the subject. 



