7 o 



LECTURES AND ESS A YS 



however, along the track of the beam a 

 beautiful sky-blue colour is observed, 

 which is due to light scattered by the 

 liberated particles of sulphur. For a 

 time the blue grows more intense ; it 

 then becomes whitish, and ends in a 

 more or less perfect white. When the 

 action is continued long enough, the 

 tube is filled with a dense cloud of sul- 

 phur particles, which by the application 

 of proper means may be rendered indi- 

 vidually visible. 1 



Here, then, our ether- waves untie the 

 bond of chemical affinity, and liberate a 

 body sulphur which at ordinary tem- 

 peratures is a solid, and which therefore 

 soon becomes an object of the senses. 

 We have first of all the free atoms of 

 sulphur, which are incompetent to stir 

 the retina sensibly with scattered light. 

 But these atoms gradually coalesce and 

 form particles, which grow larger by con- 

 tinual accretion, until after a minute or 

 two they appear as sky-matter. In this 

 condition they are individually invisible ; 

 but collectively they send an amount of 

 wave-motion to the retina, sufficient to 

 produce the firmamental blue. The 

 particles continue, or may be caused to 

 continue, in this condition for a con- 

 siderable time, during which no micro- 

 scope can cope with them. But they 

 grow slowly larger, and pass by in- 

 sensible gradations into the state of cloud, 

 when they can no longer elude the armed 

 eye. Thus, without solution of con- 

 tinuity, we start with matter in the atom, 

 and end with matter in the mass ; sky- 

 matter being the middle term of the 

 series of transformations. 



Instead of sulphurous acid, we might 

 choose a dozen other substances, and 

 produce the same effect with all of them. 

 In the case of some probably in the 

 case of all it is possible to preserve 



1 M. Morren was mistaken in supposing that 

 a modicum of sulphurous acid, in the drying 

 tubes, had any share in the production of the 

 " actinic clouds" described by me. A beautiful 

 case of molecular instability in the presence of 

 light is furnished by peroxide of chlorine, as 

 proved by Professor Dewar. 1878. 



matter in the firmamental condition for 

 fifteen or twenty minutes under the con- 

 tinual operation of the light. During 

 these fifteen or twenty minutes the 

 particles constantly grow larger, without 

 ever exceeding the size requisite to the 

 production of _the celestial blue. Now, 

 when two vessels are placed before us, 

 each containing sky-matter, it is possible 

 to state with great distinctness which 

 vessel contains the largest particles. 

 The eye is very sensitive to differences 

 of light, when, as in our experiments, it 

 is placed in comparative darkness, and 

 the wave-motion thrown against the 

 retina is small. The larger particles 

 declare themselves by the greater white- 

 ness of their scattered light. Call now 

 to mind the observation, or effort at 

 observation, made by our President, 

 when he failed to distinguish the particles 

 of mastic in Briicke's medium, and when 

 you have done this, please follow me. 

 A beam of light is permitted to act upon 

 a certain vapour. In two minutes the 

 azure appears, but at the end of fifteen 

 minutes it has not ceased to be azure. 

 After fifteen minutes its colour, and some 

 other phenomena, pronounce it to be a 

 blue of distinctly smaller particles than 

 those sought for in vain by Mr. Huxley. 

 These particles, as already stated, must 

 have been less than rsroWth of an inch 

 in diameter. And now I want you to 

 consider the following question : Here 

 are particles which have been growing 

 continually for fifteen minutes, and at 

 the end of that time are demonstrably 

 smaller than those which defied the 

 microscope of Mr. Huxley What must 

 have been the size of these particles at the 

 beginning of their growth ? What notion 

 can you form of the magnitude of such 

 particles ? The distances of stellar 

 space give us simply a bewildering sense 

 of vastness, without leaving any distinct 

 impression on the mind ; and the mag- 

 nitudes with which we have here to do, 

 bewilder us equally in the opposite direc- 

 tion. We are dealing with infinitesimals, 

 compared with which the test objects of 

 the microscope are literally immense. 



