74 THE POPULAR SCIENCE MONTHLY. 



not generally exceed that of a comet, probably ; and hence the ac- 

 tion takes place invisibly to us. Photographic surveys of the heav- 

 ens often repeated will doubtless give us more numerous records. 



We now return to the regularly condensing swarms. In these 

 the condensation will go on, and the temperature will rise until 

 the loss by radiation equals the increase of temperature due to the 

 fall of meteorites upon the continually condensing center. If we 

 imagine a star to be condensed more and more by the fall of mete- 

 oritic material upon it, we shall arrive at a time in which, pro- 

 vided that the supply of material ceases, the increase of tempera- 

 ture of the star from that reason will also cease, and then will 

 arise a condition of things in which the heat radiated from the 

 star will be greater than the heat produced in the body of gas 

 which is ultimately formed in consequence of the tremendous 

 temperature caused by the continual fall of meteoritic matter 

 toward the center. 



If it be true that in the nebulae we begin with meteoritic dust- 

 particles far separate from each other, we must gradually get an 

 increase of temperature so long as they approach nearer the center 

 of the swarm by condensation ; and so long as the heat produced 

 by bombardment is in excess of the loss by radiation, the temper- 

 ature will increase ; but when the loss by radiation exceeds the 

 gain by the bombardment we must get a reduction of tempera- 

 ture. A temperature curve like one of the arches of Westminster 

 Bridge flattened at the top will illustrate this idea. We have on 

 the left-hand arm of the curve those bodies in which we get a rise 

 of temperature due to collisions and to condensation ; along the 

 top of the curve we have the gradual formation of a globe of gas ; 

 the gas begins to cool and gradually condenses, until at the lower 

 end of the right-hand arm of the curve, as a result of the total 

 action, we get the formation of a body like the earth. 



Such a temperature curve has been provisionally divided into 

 seven parts, and what has been done so far is to show that there 

 are seven well-defined groups of bodies in space, which may be 

 located, three on the rising part of the curve, one at the top, and 

 three on the descending part ; representatives of each of these 

 groups have been classified and their spectra have been carefully 

 studied. There is absolutely no difficulty whatever about placing 

 all the celestial bodies which have been so observed by means of 

 the spectroscope in one group or the other ; and further, where 

 the spectroscopic evidence is complete, there is again no difficulty 

 in dividing these groups into species, just in the same way that 

 the biologist deals with organic forms. This has already been 

 done for one group, and in a very few years it will no doubt be 

 done for more, so that here again we are definitely in the region 

 of hard, detailed facts. 



