Curiosities of Science. 87 



In fact, this probability has almost been converted into a certainty 

 by the magnificent reflecting telescope constructed by Lord Rosse, of 

 6 feet in aperture, which has resolved, or rendered resolvable, multitudes 

 of nebulae which had resisted all inferior powers. The sublimity of the 

 spectacle afforded by that instrument of some of the larger globular and 

 other clusters is declared by all who have witnessed it to be such as no 

 words can express.* 



Although, therefore, nebulae do exist, which even in this powerful 

 telescope appear as nebulae, without any sign of resolution, it may very 

 reasonably be doubted whether there be really any essential physical 

 distinction between nebulae and clusters of stars, at least in the nature of 

 the matter of which they consist ; and whether the distinction between 

 such nebulae as are easily resolved, barely resolvable with excellent tele- 

 scopes, and altogether irresolvable with the best, be any thing else than 

 one of degree, arising merely from the excessive minuteness and multi- 

 tude of the stars of which the latter, as compared with the former, con- 

 sist. Outlines of Astronomy, 5th edit. 1858. 



It should be added, that Sir John Herschel considers the 

 " nebular hypothesis" and the above theory of sidereal aggre- 

 gation to stand quite independent of each other. 



ORIGIN OF HEAT IN OUR SYSTEM. 



Professor Helmholtz, assuming that at the commencement 

 the density of the nebulous matter was a vanishing quantity, 

 as compared with the present density of the sun and planets, 

 calculates how much work has been performed by the conden- 

 sation ; how much of this work still exists in the form of me- 

 chanical force, as attraction of the planets towards the sun, and 

 as vis viva of their motion ; and finds by this how much of the 

 force has been converted into heat. 



The result of this calculation is, that only about the 45th part of 

 the original mechanical force remains as such, and that the remainder, 

 converted into heat, would be sufficient to raise a mass of water equal to 

 the sun and planets taken together, not less than 28,000,000 of degrees 

 of the centigrade scale. For the sake of comparison, Professor Helm- 

 holtz mentions that the highest tempernture which we can produce by 

 the oxy-hydrogen blowpipe, which is sufficient to vaporise even platina, 

 and which but few bodies can endure, is estimated at about 2000 degrees. 

 Of the action of a temperature of 28,000.000 of such degrees we can 

 form no notion. If the mass of our entire system were of pure coal, 

 by the combustion of the whole of it only the 350th part of the above 

 quantity would be generated. 



The store of foixfe at present possessed by our system is equivalent 

 to immense quantities of heat. If our earth were by a sudden shock 

 brought to rest in her orbit which is not to be feared in the existing 

 arrangement of our system by such a shock a quantity of heat would 

 be generated equal to that produced by the combustion of fourteen such 

 earths of solid coal. Making the most unfavourable assumption as to 

 its capacity for heat, that is, placing it equal to that of water, the mass 



* The constitution of the nebulae in the constellation of Orion has been re- 

 solved by this instrument; and by its aid the stars of which it is composed 

 burst upon the sight of man for the first time. 



