2Q6 on comets. 



V. 



A Letter on Cimefs, addressed to Mr. Bode, Astronomer 

 Royal at Berlin, Received from the Author. 



SIR, 



Occasion of jL OU wished to have in writing the conjectures on the 

 the letter. nature of comets, which I had the honour of mentioning 

 to you in conversation a few days ago. At present however 

 they form but the embryo of a system, fully to unfold 

 which would require studies I have not pursued, particu- 

 larly that of the astronomical history of the*se bodies, and 

 of the opinions that have been entertained respecting them. 

 I cannot refuse, however, to deliver into your hands these 

 rudiments of ideas, relying on your indulgence both with 

 respect to their want of precision, and to the brevity with 

 which I treat the physical principles, on which they are 

 founded; as these principles are more fully exhibited in 

 different works of mine. But I will at least attempt to 

 its subject, explain in a general way the consequences I deduce from 

 these principles with respect to the lumbtous appearances of 

 comets ; persuaded, that, if they contain a single seed of 

 truth, your intimate acquaintance with the heavenly bodies 

 will enable you to discern and expand it. 

 All luminous I shall set out with what the whole of our knowledge of 

 substances nature appears to me to teach .us very clearly respecting the 

 gWe out light substances, that are capable of becoming luminous : this is, 



inconsequence jj lat t | ie /fo&, which then escapes from them, had entered 

 Of chemical . , ' ° . . . ,. , . ,11 



<»fcoro?osi- mto their composition as an ingredient, and is evolved by 



che m ical decomposition . 



Light of the ' In applying this principle to the heavenly bodies, observa- 



Sun from a tion has added one circumstance with respect to the Sun, 



fluid surround- ' 



i»g it. which may be extended by analogy to the fixed stars. Dr. 



Herschel, you know, has discovered, that the light, which 

 issues from the Sun, does not come directly from its solid 

 substance, but from an atmosphere, or some fluid, by which 

 Luminous it is surrounded. Thus this grand phenomenon has been 

 fcbenomena of brought nearer to us as it were ; analogy, though the sub- 

 ject* 



