﻿Intelligence and Miscellaneous Articles. 403 



perform, may be restricted is to be decided by continued expe- 

 riments which have already been commenced. The Verein has also 

 exposed two bronzes which have been artificially patinated by che- 

 mical means, in order to ascertain how they are influenced by similar 

 treatment. 



How the oil works in the formation of the patina, cannot 

 with certainty be stated. Experiments have shown that any excess 

 of oil is to be avoided, and that which has been painted on must 

 be rubbed off as much as possible. If excess of oil remains, the 

 dust deposits on it, and the bronze acquires an unsightly appear- 

 ance. It cannot be assumed that the residual small quantity of 

 oil enters into a chemical combination with the layer of oxide, 

 especially as bone-oil acts just as well as olive-oil in the expe- 

 riments. Probably the film of oil only acts in preventing the ad- 

 herence of moisture, by which dust adheres, gases and vapours are 

 absorbed, and in which vegetation forms. Whatever be the mode 

 of action, the experiments leave no doubt that the fat is essen- 

 tially connected with the formation of the patina. 



It is probable that in other respects it will be advantageous. 

 It has been observed that bronzes covered with a beautiful patina, 

 in those parts where water trickles down assume a wkite, opaque, 

 chalky surface, which in course of time is more and more washed 

 away by the water. Proper treatment with oil will doubtless pre- 

 vent the formation of these chalky places ; yet only long-continued 

 experiments can settle this point. 



In any case this use of oil justifies the hope that for the future 

 we may retain beautifully patinated monuments even in large towns. 

 Where coal is the only combustible, they will not be bright, but 

 dark green, and perhaps even black ; but they will have the other 

 beautiful property of the patina, the peculiar transparent condi- 

 tion of the surface. — Poggendorff's Annalen, April 1869. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, Streatham Hill, April 8, 1869. 



Professor Tyndall's interesting cometary theory, which appeared 

 in your last Number, turns on an assumption which is hardly com- 

 patible with the laws of motion. His assumption is that the tail of 

 a comet is " matter precipitated on the solar beams traversing the 

 cometary atmosphere." If so, the linear dimensions of a comet 

 must exceed the length of its tail — that is, in some cases 60 millions 

 of miles or more. Now it seems utterly incredible that such a bulk of 

 vapour can whirl round the sun at the perihelion passage unbroken. 

 A very slight modification of Tyndall's theory avoids this difficulty, and 

 equally well explains all the observed appearances. If there be an 

 extremely attenuated solar atmosphere extending considerably fur- 

 ther than the earth's orbit, and if, when a comet approaches the sun 

 and is therefore exposed to intense heat, its volume becomes compa- 

 rable with the volume of the sun, the heat of the sun will be shut off 

 from that portion of the solar atmosphere in the shade of the comet 

 (which though transparent to light is opaque to heat), and actinic 



