CHEMICAL SCIENCE. 165 



water-type in connection with poly-atomic as -well as mon-atomic radi- 

 cles, -was found to satisfy the requirements of the synoptic formulae. 

 Gerhardt was the first to adopt them from us. He gaye a system of 

 organic chemistry on that plan, and his book has been of immense ser- 

 vice to the deyelopmeut of our science. The extension of these prin- 

 ciples to mineral chemistry had been commenced in the cases of the 

 commonest acids and bases, but their general introduction met with 

 difficulties, and sometimes seemed wanting to their complete success. 

 It was reserved for Prof. Cannizzaro, of the University of Palermo, to 

 show us how the remainder of the knot could be untied. He argued, 

 upon physical as well as chemical grounds, that the atomic weight of 

 many metals ought to be doubled, as well as those of oxygen, sulphur, 

 and carbon. His conclusions are confirmed by the constitution of those 

 organo-metallic bodies which I mentioned just now, and it certainly 

 does seem to supply what was still wanting for the extension of our 

 system of classification from the non-metallic elements to the heavy 

 metals themselves. The elements are now arranged into two principal 

 groups : 1 . Those of which each atom combines with an uneven num- 

 ber of atoms of chlorine or hydrogen. 2. Those of which each atom 

 combines with an even number of atoms of chlorine or hydrogen. 

 Like every classification founded upon nature this one draws no abso- 

 lute line, as some elements belong to both classes. The first group in- 

 cludes the mon-atomic elements of the chlorine family, the tri-atomic 

 elements of the nitrogen family, hydrogen, and the alkali metals, silver 

 and gold, - - in all about eighteen elements. The usual atomic weights 

 of these are retained. The usual atomic weights of all the other ele- 

 ments, biatomic. tetratomic, etc., are doubled. This second group in- 

 cludes the oxygen family, carbon, silicon, and the alkaline earths, the 

 metals zinc, iron, copper, lead, etc. Every step in our theoretical de- 

 velopment of chemistry has served to consolidate and extend the atomic 

 theory, but it is interesting to observe that the retention of that theory 

 has involved the necessity of depriving it of the absolute character which 

 it at first possessed. Organic compounds were long ago discovered, 

 containing atoms of carbon, hydrogen, and oxygen in proportions far 

 from simple ; and the atomic theory must have been abandoned, but for 

 the discovery that the atomic, or rather molecular, weights of these 

 compounds correspond invariably to entire numbers of the elementary 

 atoms. We now use the term " molecule " for those groups which hold 

 together during a variety of transformations, but which can be resolved 

 into simpler constituents ; whilst we reserve the word " atom " for those 

 particles which we cannot break up, and which there is no reason for 

 believing that we ever shall break up. 



RELATION OF SCIENTIFIC RESEARCH TO MEDICAL SCIENCE. 



At the last meeting of the British Association, several examples were 

 brought forward demonstrating the direct bearing of scientific research- 

 es upon the advance of medical science. No sooner is any new sub- 

 stance (whether an elementary body, such as thallium, or a compound) 

 discovered, than experiments are made to investigate its physiological and 

 therapeutical action on the living organisms of men and animals. In 

 many cases, these experiments are made by the observers on their own 

 bodies, and the records of science offer several examples of enthusiastic 



