240 



NA TURE 



\_yuly 30, 1874 



his youngest son, and here he himself died on Feb. 6, 



1804. 



Turning now from the external aspects of Priestley's life, 

 let us consider the position he held as a philosopher and 

 the influence that his discoveries had on the science of his 

 time. The ever-memorable discovery of " dephlogisticated 

 air" on Aug. i, 1774, marks an epoch in the annals of 

 chemistry with which the name of Dr. Joseph Priestley 

 will be always associated. He obtained it by exposing a 

 quantity of red precipitate of mercury to the action of the 

 sun's rays concentrated upon it by a lens ; the red preci. 

 pitate was contained in a flask filled up with mercury and 

 inserted in a basin containing the same metal. " I pre- 

 sently found," he says, " that by means of this lens air 

 was expelled from it very readily. Having got several 

 times as much as the bulk of my materials, I admitted 

 water to it, and found that it was not imbibed by it ; but 

 what surprised me more than I can well express, was that 

 a candle burned in this air with a remarkably vigorous 

 flame very much like that enlarged flame with which a 

 candle burns in nitrous air exposed to iron or lead of sul- 

 phur ; but, as I got nothing like this remarkable appear- 

 ance from any kind of air besides this particular modifica- 

 tion of nitrous air, and 1 knew no nitrous acid was used 

 in the preparation of incrciirius cakinatiis, I was utterly 

 at a loss how to account for it." He then goes on to 

 show that red lead and nitre also afford oxygen at a red 

 heat, and calls it, consistently with the theory of combus- 

 tion which was then prevalent, dephlogisticated air, 

 regarding it as common air deprived of phlogiston, and 

 consequently possessed of a powerful affinity for that 

 imaginary principle. 



This discovery, however, though unquestionably bril- 

 liant, must not be allowed to eclipse those other 

 numerous and ^valuable contributions to science with 

 which this indefatigable worker enriched the stores of 

 natural knowledge during a period ranging from 176S to 

 1800. There are indeed few branches of natural science 

 which did not reap some benefit, direct or indirect, from 

 the discoveries of the experimenter whose memory we 

 now recall. 



On the 17th of August, 1771, Priestley enclosed a sprig 

 of mint in air in which a taper had been allowed to burn 

 out, and he found on the 27th of the month that the same 

 air'then permitted the combustion of another taper with 

 perfect facility. Thus was the secret of vegetable respira- 

 tion first made known. In the discoverer's own words : 

 " This restoration of air I found depended upon the vege- 

 tating state of the plant ; for though 1 kept a great number 

 of the fresh leaves of mint in a small quantity of air in 

 which candles had burned out, and changed them fre- 

 quently for a long space of time, I could perceive no 

 melioration in the state of the air." In pneumatic 

 chemistry (of which the germs had been originated by 

 Black, Mayow, Hooke,and Hales), Priestley found a new 

 engine of research, and in his hands this Opyavov yielded 

 vast results. His productions in pure chemistry are too 

 well known to be discussed fully here, even did space 

 permit. In addition to oxygen he discovered nitrous 

 oxide (1776), sulphurous anhydride (1774), ammonia gas 

 (1774), carbonic oxide and hydrochloric acid gas (1772) : 

 he was also the first to investigate the properties of nitric 

 oxide. We may point to nitrous oxide en passant as one 



of the many instances in which pure science has furnished 

 a substance of practical utility to man : the discoverer of 

 " dephlogisticated nitrous air " little dreamt that the lapse 

 of a century would see this substance used as an anccs- 

 thetic for the purposes of dentistry. The pneumatic and 

 mercurial troughs, now indispensable parts of our labora- 

 tory " plant," were also bequeathed to us by the philo- 

 sopher of Fieldhead. Although chemistry received the 

 greater part of Priestley's attention, other branches of 

 science, as before stated, received the benefit of his 

 thoughts. Thus we find a work by him bearing the date 

 1772, entitled "The History and Present State of Disco- 

 veries relating to Vision, Light, and Colours," and we have 

 already referred to his " History of Electricity." From 

 a catalogue of Priestley's works, printed at the end ot 

 his " Experiments and Observations relating to various 

 branches of Natural Philosophy," we find that this extra- 

 ordinary man was the author of no less than thirty-six 

 volumes on various subjects ; among others, the theory 

 and practice of perspective, charts of history and bio- 

 graphy, rudiments of grammar, observations on educa- 

 tion, a course of lectures on oratory and criticism, an 

 essay on the first principles of government, and on the 

 nature of political, civil, and religious liberty, together 

 with large numbers of works on metaphysical subjects 

 and on theology. 



But it is with the clicmical aspect of Priestley's life that 

 we are more particularly concerned at present. The 

 anniversary about to be celebrated is that of a purely 

 chemical discovery, and one which to us appears doubly 

 important, first, from the great flood of light which it shed 

 on the processes of combustion and of respiration, both 

 animal and vegetable, aerial and aquatic ; and secondly^ 

 from the powerful illustration which it affords of the value 

 of a new method in scientific investigation. The purely 

 practical I'esults which in after years flowed from the dis- 

 covery of oxygen, such, for example, as the oxy-hydrogen 

 blowpipe, which enables large quantities of platinum and 

 of the most refractory metals to be smelted with ease, are 

 at present of minor interest. Is it not this over-anxious 

 regard for " practical results " that has led to the com- 

 plaints, too frequently made, about the decline of chemical 

 research in England ? The spirit of the old investigators 

 of the school of Priestley, Cavendish, and Black seems to 

 be forsaking us, and, with certain exceptions, our most 

 efficient workers are devoting their time and energies to 

 eftecting permutations and combinations among the ele- 

 ments—in seeing in how many ways certain atoms of 

 carbon, hydrogen, and oxygen can be combined, or in 

 locating atoms to certain imaginary positions in space. 

 It must not be for a moment supposed that we ad- 

 vocate the entire cessation of this kind of work — 

 it is useful in its way as supplying facts, but by itself it is 

 not sufficient to lead us to hope for any great advance- 

 ment in our knowledge of chemical laws. The greatest 

 advancements in chemistry have been the results of the 

 application of pliysical discoveries — witness the vapour- 

 density control for the formula; of compounds and the 

 atomic weights of the elementary gases ; or the determi- 

 nation of specific heat as a means of controlling the 

 atomic weight ; or turn again to that great engine of 

 modern research, the spectroscope, which has enabled us 

 to extend our list of known elements, and which reduces 



