The Evolution of Chemistry. 133 



take one element as a standard of reference for the rest. 

 Oxygen was chosen by some and hydrogen by others, while 

 some scattering chemists tried plans of their own. It was 

 finally seen that the lightest element of all should be chosen 

 for a standard, while the rest were adjusted in relation to 

 it. Hydrogen, being the lightest, became the standard of 

 reference. When oxygen is said to weigh sixteen, the mean- 

 ing is that it is sixteen times as heavy as hydrogen. When 

 sulphur is placed at thirty-two, we are to understand that 

 that substance has an atom thirty-two times the weight of 

 an atom of hydrogen. In early determinations only rough 

 approximations were made toward the true figures and, be- 

 fore the acceptance of Avogadro's law, figures, were often 

 chosen that were not true atomic weights but only ratios 

 thereof. In water they found eight times more weight of 

 oxygen than hydrogen, and so the heavier element was put 

 down as u eight" ATOgadro showed that since it took two 

 volumes of hydrogen to saturate one of oxygen in forming 

 water, therefore there must be two atoms of hydrogen 

 to one of oxygen. This lowered the comparative weight of 

 hydrogen one half, so that oxygen had to be called sixteen 

 instead of eight Many changes of this kind had to be in- 

 troduced. The old formula for water was HO, but the new 

 one is H,0. In the first the relationship was as one to 

 eight, but in the second it is two to sixteen. In writing 

 chemical formulae the letters stand for atoms of the ele- 

 ments, H means one atom of hydrogen. H, means two 

 atoms of hydrogen. H 8 means one molecule of water con- 

 taining two atoms of hydrogen and one of oxygen. 5H 8 

 would mean five molecules of water. The study of the laws 

 of heat that was going onparipassu with the development 

 of chemistry led Bulong and Petit in 1819 to make a very 

 remarkable discovery. When bodies are wanned it is found 

 that the amount of heat required to raise them one degree 

 varies very materially among them. Taking water as unity 

 and calculating the relation for other substances, we get 

 what is called their specific heat These physicists found 

 that for thirteen elements which they had tried, the specific 

 heats were inversely proportional to their atomic weights. 

 This meant directly proportional to the number of atoms 

 present. Here then was a new means of determining atomic 

 weights confirmatory and supplementary to the law of 

 Avogadro. Although later investigations showed limiting 

 conditions to the law, it has been successfully used in settling 



