218 PRINCIPLES OF CHEMISTRY 



atoms and m and n their relative number. If the same elements A and 

 B, in addition to A M B m . also yield another compound A,.B <; , then by- 

 expressing the composition of the first compound by A lir B mr (and this 

 is the same composition as A H B m ), and of the second compound by 

 A ru B 5n , we have the law of multiple proportions, because for a given 



of gases cannot be expected. Therefore the absence of an atmosphere about the moon, 

 if it were not subject to doubt, would be counted as a forcible proof of the atomic 

 theory. As a proof of the absence of a lunar atmosphere, it is cited that the moon, 

 in its independent movement between the stars, when eclipsing a star that is, when 

 passing between the eye and the star does not show any signs of refraction at its 

 edge ; the image of the star does not alter its position in the heavens on approach- 

 ing the moon's surface, consequently there is no atmosphere on the moon's surface 

 capable of refracting the rays of light. Such is the conclusion by which the absence of 

 a lunar atmosphere is acknowledged. But this conclusion is most feeble, and there are 

 even facts in exact contradiction to it, by which the existence of a lunar atmosphere 

 may be proved. The entire surface of the moon is covered with a number of mountains, 

 having in the majority of cases the conical form natural to volcanoes. The volcanic 

 character of the lunar mountains was confirmed in October 1866, when a change was 

 observed in the form of one of them (the crater Linnea). These mountains must be on 

 the edge of the lunar disc. Seen in profile, they screen one another and interfere with 

 -making observations on the surface of the moon, so that when looking at the edge of 

 the lunar disc we are obliged to make our observations not on the moon's surface, but 

 at the summits of the lunar mountains. These mountains are higher than those on 

 our earth, and consequently at their summits the lunar atmosphere must be exceed- 

 ingly rarefied even if it possess an observable density at the surface. Knowing the mass of 

 the moon to be eighty-two times less than the mass of the earth, we are able to approxi- 

 mately determine that our atmosphere at the moon's surface would be about twenty- 

 eight times lighter than it is on the earth, and consequently at the very surface of the 

 moon the refraction of light by the lunar atmosphere must be very slight, and at the 

 heights of the lunar mountains it must be imperceptible, and would be lost within the 

 limits of experimental error. Therefore the absence of refraction of light at the edge of 

 the moon's disc cannot yet plead in favour of the absence of a lunar atmosphere. There 

 is even a series of observations obliging us to admit the existence of this atmosphere, 

 These researches are due to Sir John Herschel. This is what he writes : ' It has often 

 been remarked that during the eclipse of a star by the moon there occurs a peculiar 

 optical illusion; it seems as if the star before disappearing passed over the edge of the 

 moon and is seen through the lunar disc, sometimes for a rather long period of time. I 

 myself have observed this phenomenon, and it has been witnessed by perfectly trust- 

 worthy observers. I ascribe it to optical illusion, but it must be admitted that the star 

 might have been seen on the lunar disc through some deep ravine on the moon.' Geniller, 

 in Belgium (1856), following the opinion of Kassine, Eiler, and others, gave an explana- 

 tion to this phenomenon ; he considers it due to the refraction of light in the valleys of 

 the lunar mountains which occur on the edge of the lunar disc. In fact, although 

 these valleys do not probably present the form of straight ravines, yet it may sometimes, 

 happen that the light of a star is so refracted that its image might be seen, notwith- 

 standing the absence of a direct path for the light-rays. He then goes on to remark 

 that the density of the lunar atmosphere must be variable in different parts, owing to 

 the very long nights on the moon. On the dark, or non-illuminated, portion, owing to 

 these long nights, which last thirteen of our days and nights, there must be excessive cold, 

 and hence a denser atmosphere, while, on the contrary, at the illuminated portion the 

 atmosphere must be much more rarefied. This variation in the temperature of the 

 different parts of the moon's surface explains also the absence of clouds, notwithstanding 

 the possible presence of air and aqueous vapour, on the visible portion of the moon. The 



