122 - FRAGMENTS OF SCIENCE. 



vation, 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 dis- 

 tinctly smaller particles than those sought for in vain 

 by Mr. Huxley. These particles, as already stated, 

 must have been less than 100 1 000 th of an inch in dia- 

 meter. And now I want you to consider the following 

 question : Here are particles which have been growing 

 continually for fifteen minutes, and at the en.d of that 

 time are demonstrably smaller than those wliich 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 

 magnitudes with which we have here to do, bewilder 

 us equally in the opposite direction. We are dealing 

 with infinitesimals, compared with which the test 

 objects of the microscope are literally immense. 



From their perviousness to stellar light, and other 

 considerations, Sir John Herschel drew some startling 

 conclusions regarding the density and weight of comets. 

 You know that these extraordinary and mysterious 

 bodies sometimes throw out tails 100,000,000 miles in 

 length, and 50,000 miles in diameter. The diameter of 

 our earth is 8,000 miles. Both it and the sky, and a 

 good portion of space beyond the sky, would certainly 

 be. included in a sphere 10,000 miles across. Let us 

 fill a hollow sphere of this diameter with cometary 



