182 History of Luminescence 



found to proceed from that celebrated chymical production called phos- 

 phorus, which is, in fact, an imperfect sulphur, tending to decompose 

 itself, and so as to take fire by the access of air only. Phosphorus, there- 

 fore, when it emits light is properly a body ignited; though when a very 

 small quantity of it is used, as what is left after drawing it over paper, 

 or what may be dissolved in essential oil, the heat is not sensible. But 

 perhaps the matter which emits the light in what we call putrescent 

 substances may be familiar to it though it be generated by a different 

 process, and burn with a less degree of heat. Putrescence does not seem 

 to be necessary to the light of glow-worms, or of the pholades: and yet 

 their light is sufficientily familiar to that of shining wood or flesh. Elec- 

 tric light is unquestionably similar to that of phosphorus, though the 

 source of it is apparently very different. 



It is apparent that, in Vision, Light and Colours, Priestley was 

 greatly influenced by Newton. In his sinnmary he remarked that 

 the observations made in his book " will authorize us to take it for 

 granted that light consists of very minute particles of matter, emitted 

 from luminous bodies." In the " Hints of some Desiderata," a chap- 

 ter proposing new subjects of inquiry, the Queries of Newton are 

 quoted and discussed in detail. 



Priestley is undoubtedly best known for his study of oxygen and 

 other gases. In Section VIII, at the end of the first volume of Experi- 

 ments and Observations on Dij]erent Kinds of Air (London, 1774) , 

 in a section entitled " Queries, Speculations and Hints," Priestley 

 speculated on a number of puzzling phenomena. He stated that 

 electric matter directed through the body of any muscle forces it to 

 contract and held: 



that the source of muscular motion is phlogiston [the fire principle] is 

 still more probable, from the consideration of the well known effects of 

 vinous and spirituous liquors, which consist very much of phlogiston, and 

 which instantly brace and strengthen the whole nervous and muscular 

 system. . . . 



As to the manner in which the electric inatter makes a muscle contract 

 I do not pretend to have any conjecture worth mentioning. . . . Possibly, 

 the light which is said to proceed from some animals, as from cats and 

 wild beasts, when they are in pursuit of their prey in the night, may 

 not only arise, as it has hitherto been supposed to do, from the mere 

 friction of their hairs and bristles; etc. but that violent muscular exer- 

 tion may contribute to it. This may assist them occasionally to catch 

 their prey; as glow-worms, and other insects, are provided with a constant 

 light for that purpose, to the supply of which light their nutriment may 

 also contribute. 



Priestley then Avent on to apply this statement to hirnian beings: 



