SCIENTIFIC USE OF THE IMAGINATION 



69 



exactly the condition to give us the glow 

 of the Alps, but it was a step in this 

 direction. Briicke's fine precipitate, above 

 referred to, looks yellowish by transmitted 

 light ; but, by duly strengthening the 

 precipitate, you may render the white 

 light of noon as ruby-coloured as the 

 sun, when seen through Liverpool smoke 

 or upon Alpine horizons. I do not, 

 however, point to the gross smoke arising 

 from coal as an illustration of the action 

 of small particles, because such smoke 

 soon absorbs and destroys the waves of 

 blue, instead of sending them to the eyes 

 of the observer. 



These multifarious facts, and number- 

 less others which cannot now be referred 

 to, are explained by reference to the 

 single principle, that, where the scatter- 

 ing particles are small in comparison 

 to the ethereal waves, we have in the 

 reflected light a greater proportion of 

 the smaller waves, and in the trans- 

 mitted light a greater proportion of 

 the larger waves, than existed in the 

 original white light. The consequence, as 

 regards sensation, is that in the one case 

 blue is predominant, and in the other 

 orange or red. Our best microscopes 

 can readily reveal objects not more than 

 Tuiiroth of an inch in diameter. This 

 is less than the length of a wave of red 

 light. Indeed, a first-rate microscope 

 would enable us to discern objects not 

 exceeding in diameter the length of the 

 smallest waves of the visible spectrum. 1 

 By the microscope, therefore, we can 

 test our particles. If they be as large as 

 the light-waves, they will infallibly be 

 seen ; and if they be not so seen, it is 

 because they are smaller. Some months 

 ago I placed in the hands of our Presi- 

 dent a liquid containing Briicke's pre- 

 cipitate. The liquid was milky blue, and 

 Mr. Huxley applied to it his highest 

 microscopic power. He satisfied me that, 

 had particles of even rstWrrth of - an 

 inch in diameter existed in the liquid, 



1 Dallinger and Drysdale have recently 

 measured cilia T&TjVijtfth of an inch in diameter. 

 1878. 



they could not have escaped detection. 

 But no particles were seen. Under the 

 microscope the turbid liquid was not to 

 be distinguished from distilled water. 1 



But we have it in our power to imitate, 

 far more closely than we have hitherto 

 done, the natural conditions of this prob- 

 lem. We can generate, in air, artificial 

 skies, and prove their perfect identity 

 with the natural one, as regards the exhi- 

 bition of a number of wholly unexpected 

 phenomena. By a continuous process of 

 growth, moreover, we are able to connect 

 sky-matter, if I may use the term, with 

 molecular matter on the one side, and 

 with molar matter, or matter in sensible 

 masses, on the other. In illustration ot 

 this, I will take an experiment suggested 

 by some of my own researches, and 

 described by M. Morren of Marseilles at 

 the Exeter meeting of the British Asso- 

 ciation. Sulphur and oxygen combine 

 to form sulphurous acid gas, two atoms 

 of oxygen and one of sulphur constitut- 

 ing the molecule of sulphurous acid. It 

 has been recently shown that waves of 

 ether issuing from a strong source, such 

 as the sun or the electric light, are com- 

 petent to shake asunder the atoms of 

 gaseous molecules. 2 A chemist would 

 call this " decomposition " by light ; but 

 it behoves us, who are examining the 

 power and function of the imagination,, 

 to keep constantly before us the physical- 

 images which underlie our terms. There- 

 fore I say, sharply and definitely, that 

 the components of the molecules of 

 sulphurous acid are shaken asunder by 

 the ether-waves. Enclosing sulphurous 

 acid in a suitable vessel, placing it in a 

 dark room, and sending through it a 

 powerful beam of light, we at first see 

 nothing : the vessel containing the gas 

 seems as empty as a vacuum. Soon, 



'Like Dr. Burdon Sanderson's "pyrogen," 

 the particles of mastic passed, without sensible 

 hindrance, through filtering-paper. By such 

 filtering no freedom from suspended particles is 

 secured. The application of a condensed beam 

 to the filtrate renders this at once evident. 



2 See article on "New Chemical Reactions 

 Produced by Light," Fragments o/ Science, vol. i. 



