SIR WILLIAM RAMSAY — MOUREU. 533 
We will leave here various other works, of a special nature, in or- 
der to come without more delay to those researches which were to 
immortalize the name of Ramsay. 
In 1894 Ramsay was 42 years of age. His work was already con- 
siderable in amount and his reputation solidly established, but he 
could not yet be called a celebrity. In possession of scientific knowl- 
edge as profound as it was extensiA'e and varied, a penetrating mind 
with broad vision, a philosopher mindful of the general movement 
of the sciences, and eager to solve the mysteries of nature, free from 
all dogmatism and with mind open to even the most daring concep- 
tions, an experimenter of finished technique, an enthusiastic spirit, 
Ramsay was ready for epoch-making discoveries. Given a favorable 
occasion, his genius would be fully equal to the task. Here is the 
occasion. 
As often happens in scientific research, a chance observation may 
lead to the most unexpected results. Lord Rayleigh, who for sev- 
eral years had pursued with meticulous care the determination of 
the density of the principal simple gases (hydrogen, oxygen, nitro- 
gen), noticed that the density of the nitrogen extracted from the 
air through absorption by other known gases was always greater 
than that of chemical nitrogen, coming from different sources — 
oxides of nitrogen, ammonia, urea, etc. The difference affected the 
third decimal and did not exceed one-half per cent, but it was cer- 
tainly more than experimental error. 
Three hypotheses could explain this irregularity. The atmospheric 
nitrogen might be constituted in part of complex molecules of nitro- 
gen comparable to the oxygen compound called ozone. Conversely, 
in the chemical nitrogen a certain proportion of the molecules might 
be dissociated into free atoms. But the density of neither of the 
gases, after being kept for eight months, underwent any change, and 
the permanent existence of condensed nitrogen or of dissociated 
nitrogen (atomic nitrogen) would scarcely be likely. Lord Ray- 
leigh, who had at first accepted these explanations, rejected them to 
adopt the third hypothesis, according to which the amospheric nitro- 
gen is constituted of a chemical nitrogen mixed with an unknown gas 
of greater density. Being consulted by Lord Rayleigh, Ramsay was 
of the same opinion, and the two scholars at once united their efforts 
to isolate the mysterious gas whose existence was thus revealed. 
It is interesting to recall here that in the fundamental experiments 
in which Cavendish, a century before, had established the formation 
of nitric acid by the prolonged action of electric sparks on a mixture 
of oxygen and nitrogen in moisture, the celebrated English chemist 
had noted that even after a very long time there always remained 
after absorption of the oxygen in excess a small gaseous residue rep- 
12573°— 21 35 
