COLLOIDS 229 



as a practical instrument less than seventy years ago, has 

 enabled the astronomer and the chemist to penetrate and to 

 investigate the composition as well as the motions of these 

 distant sources of light. The result has been to show that the 

 elements which enter into the composition of the earth, the 

 seas, and the atmosphere of our planet, are to be found every- 

 where in the most distant regions of space. They are not equally 

 distributed, for in one star hydrogen, for example, may be found 

 to be present, while in another the lines indicating that element 

 are absent, and similar concentrations of calcium and magnesium, 

 iron, sodium, and the rest may be recognised in stars and 

 nebula?. 



This, however, is not the direction in which attention must be 

 concentrated in these pages. The reader who is interested in 

 astro-physics or astro-chemistry must consult one of the 

 numerous treatises on the subject, from which he may learn 

 something of the progress which has been made in spectroscopic 

 investigation concerning the composition of our sun and the 

 stars, which are believed to be constituted like our sun, as well 

 as the nature of comets and nebulae. It is sufficient to say that 

 the results of late years have been to some considerable extent 

 employed in connection with theories concerning the origin and 

 evolution of the elements, a subject which has already been dealt 

 with in Chapter VIII. 



Systematic and quantitative chemistry is based on the assump- 

 tion that matter is not divisible ad infinitum. It is known to 

 the senses in the form of masses, the most minute of which, 

 discernible by sight, contain many millions of the ultimate 

 particles molecules or atoms which are subjects of study by 

 the physicist and chemist. This is true even of the granules, 

 which, produced by the subdivision of ordinary massive matter, 

 are large enough to be just visible under the highest powers of 

 the ordinary microscope. Particles which are too small to be seen 

 by the eye alone or assisted by lenses may be perceived by other 

 senses, especially by smell. Everyone has heard of the grain of 

 musk which, lying for years on the pan of a balance, continues 

 to emit its characteristic odour without betraying any loss of 

 substance by appreciable loss of weight. And yet there seems 

 no reason to doubt that the effect on the olfactory surface is 

 produced by contact with the minute particles of musk sub- 

 stance which enter the organ. The sense of smell, like that of 



