544 SPECIAL METHODS OF DISCOVERT. 



gases, and to discover the approximate degree of electric 

 conduction resistance o/ liquid carbonic anhydride. Warl- 

 tire's experiment of exploding a mixture of common air 

 and hydrogen in a closed vessel, in order to ascertain 

 whether heat was a ponderable substance, suggested to 

 Cavendish his experiment, by means of which he dis- 

 covered the composition of water by synthesis. After 

 Scheele had discovered that nitrate of silver turns black in 

 the violet rays of the spectrum, Kitter, in the year 1801, 

 by repeating the experiment, found that the greatest 

 blackening effect took place not in the yellow or brightest 

 part of the spectrum, nor even in the visible part at all, 

 but at a little distance beyond the violet end. This was 

 the origin of photography. By making some improve- 

 ments in the apparatus employed by Magnus for testing 

 the transparency of gases to radiant heat, Buff and Hoor- 

 weg were enabled to discover that air containing aqueous 

 vapour was but little more opaque than air alone, and 

 much less so than it had been previously supposed to be. 1 



b. By extending the researches of others. Very few 

 scientific researches are entirely novel. In nearly all of 

 them something has already been done ; and in many the 

 original research of one man is only extended by another 

 either by means of additional experiments of a similar 

 kind, or by extending the same research in some par- 

 ticular direction ; and indeed the whole realm of new 

 knowledge may be considered as being evolved by one 

 vast research, of which different investigators develop 

 particular branches. We require to stand upon the terra 

 firma of the known, in order to be able to acquire a view 

 of the unknown. 



After one man has discovered an apparently singular 

 property in a substance, either himself or others discover 



1 See Philosophical Magazine, December 1877, pp. 423, 424. 



