528 SCIENCE PROGRESS. 



simple, or even the simplest form of matter, which forms 

 itself into atoms just as these atoms in turn unite to build up 

 what we call compounds ? This idea was much encouraged 

 by a remarkable letter published in 1815 in Thomson's 

 Annals of Philosophy. The theory advanced was that the 

 weights of the atoms of all elementary substances were 

 multiples of the weight of an atom of hydrogen. The first 

 letter (2) was followed by a second, both published anony- 

 mously, although it was known shortly after that both were 

 by Dr. W. Prout, and the hypothesis received the name of 

 Prout's Theory, or more often, Prout's Law. It was re- 

 ceived with great favour by chemists in this country and in 

 France, but by Berzelius and the German chemists it was 

 looked on with suspicion and distrust. Chemists were thus 

 divided into two schools, and to the friendly rivalry between 

 these we are indebted for many pieces of classical research 

 in this department, and for great improvements in the 

 ordinary processes of quantitative analysis. Amongst the 

 first consequences of the increased accuracy of quantitative 

 determinations was the proof that the atomic weight of 

 chlorine could not be regarded any longer as thirty-six 

 times that of hydrogen but only thirty-five and a half, and 

 from this it was evident that the atom of hydrogen as a 

 whole could no longer be taken as the unit for Prout's 

 Theory, and having got down to half an atom it was but a 

 little step further to get down to a quarter of an atom. In 

 this second letter Prout remarks that "we may almost con- 

 sider the -npoiTT] vXr) of the ancients to be realised in hydro- 

 gen " but fifteen years later (3) he says that " there seems to 

 be no reason why bodies still lower in the scale than hydro- 

 gen (similarly, however, related to one another, as well as 

 to those above hydrogen) may not exist, of which other 

 bodies may be multiples, without being actually multiples of 

 the intermediate hydrogen". It is now generally admitted 

 that Prout's Law must be given up altogether in its original 

 sense, but it must be very clearly borne in mind that this by 

 no means involves the giving up of such views as to the ulti- 

 mate constitution of matter as those which were held by the 

 chemists who gave the theory a hearty welcome. Nor does 



