468 THE POPULAR SCIENCE MONTHLY, 



erfiil pump. In the liigber mollusks the heart has generally two 

 cavities an auricle for receiving the blood and a ventricle for pro- 

 pelling it. The bivalve mollusks have generally two auricles. In 

 the mollusks we discover a well-developed capillary system, but the 

 venous or return circulation is still partly lacunar. The heart of in- 

 vertebrates is always systemic it forces the blood to the body, not to 

 the breathing-organs. But some of the cephalopod mollusks, the so- 

 called devil-fishes, have contractile cavities at the bases of the gills, 

 which act the part of a pulmonary heart, forcing the blood through 

 the breathing-organs on its way to the true heart. These accessory 

 hearts are called branchial hearts. 



[To be continued.] 



-*^^~ 



THE TEACHINGS OF MODEEN SPECTEOSCOPY.* 



By Dr. ARTHUR SCHUSTER, F. R. S. 



A SCIENCE, like a child, grows quickest in the first few years of 

 its existence ; and it is therefore not astonishing that, though 

 twenty years only have elapsed since Spectrum Analysis first entered 

 the world, we are able to speak to-day of a modern spectroscopy, with 

 higher and more ambitious aims, striving to obtain results which shall 

 surpass in importance any of those achieved by the old spectroscopy, 

 to the astonishment of the scientific world. 



A few years ago the spectroscope was a chemical instrument. It 

 was the sole object of the spectroscopist, to find out the nature of a 

 body by the examination of the light which that body sends out when 

 it is hot. The interest which the new discovery created in scientific 

 and unscientific circles was due to the apparent victory over space 

 which it implied. No matter whether a body is placed in our labora- 

 tory or a thousand miles away at the distance of the sun or of the 

 farthest star as long as it is luminous and sufficiently hot, it gives us 

 a safe and certain indication of the elements it is composed of. 



To-day, we are no longer satisfied to know the chemical nature of 

 sun and stars ; we want to know their temperature, the pressure on 

 their surface ; we want to know whether they are moving away from 

 us or toward us ; and, still further, we want to find out, if possible, 

 what changes in their physical and chemical properties the elements 

 with which we are acquainted have undergone under the influence of 

 the altered conditions which must exist in the celestial bodies. Every 

 sun-spot, every solar prominence, is a study in which the unknown 

 quantities include not only the physical conditions of the solar surface, 



* Address delivered at the Royal Institution of Great Britain, January 28, 1881. 



