OBSERVATIONS OF LUNAR OBJECTS. 255 



varies its tint to the greatest extent. Tliere are further phenomena which 

 require explanation ; neither the light nor the dark tints are seen at sunrise 

 or sunset, but a greenish tint characterizes the floor at those times. It is 

 when the sun attains an altitude of 30° that both the light and dark tints 

 appear ; and it has been especially noted that when the craterlets assume 

 the appearance of white spots, the sun is usually about 30° high. All the 

 phenomena hitherto observed on Plato, except the variations in the visibility 

 of the spots, and, it may be, in the visibility of the streaks also, depend upon 

 solar influence. 



In order to guide future inquiry, it may probably be useful to present an 

 enunciation of the principal features of the hypothesis employed in explain- 

 ing the above-recorded variations as bearing upon the lighter and darker 

 markings of lunar plains generally. 



The hypothesis is based upon the Icnoiun properties of gases and their 

 affections by heat. Being weU acqiiainted with phenomena, the proximate 

 causes of which are understood, we may proceed to the study of other pheno- 

 mena of which the loci are inaccessible to us, but which, being hnotvable, 

 we may also, by observation and induction, become acquainted with their 

 causes. 



We Jcnoiu that the effect of heat on all bodies whatever is to vaporize them, 

 and this vaporization proceeds at all temperatures, low as well as high. We 

 also know that vapours behave as permanent gases, are diffused through 

 them, are elastic like them, and are expanded as they are by successive in- 

 crements of heat. We further l-now that vapours of even solid substances 

 attain a state of maximum density in given volumes of gases dependent upon 

 temperature ; and our hnowledge extends a step further, viz. that when the 

 temperature of a given volume of gas is diminished below the point of 

 maximum density of any particular vapour, the superabundant vapour is 

 condensed and cloud or dew are formed, and this alike of metallic as well as 

 of liquid substances. Now, bearing in mind these four results cognizant by 

 us, the conclusion seems to be irresistible, — (1) that the sun shining on the 

 moon's surface must vaporize the materials of which it is composed ; (2) that 

 the vapours thus raised from the surface must be dissimilar, inasmuch as 

 the different reflective powers of different parts of the surface indicate the 

 existence of different materials composing the surface ; (3) that the different 

 vapours resting on the solid surface act upon each other and upon the ma- 

 terials of the surface itself, so that diffusion takes place, and maximum den- 

 sities are attained as the temperature both of the surface and of the vapours 

 increase ; (4) that the expansibility of the vapours raised above the surface 

 by the accumulated heat of at least 177 hours of uninterrupted sunshine 

 must produce ascensional currents of the liberated vapours, carrying them 

 into colder regions, where condensation occurs, and cloud or mist is formed ; 

 (.5) that the attraction of gravitation acting on the condensed vapours caiises 

 them to descend into warmer regions, where they are dissolved ; and as the 

 temperature declines less vapour is raised, and the features of the surface 

 become unobscured. 



A very pertinent question may here be asked. Is this hypothesis capable 

 of substantiation, or, upon examination, is it likely to be found destitute of 

 proof? In rej)ly it may be asked. Do the darker markings, which appear to 

 be formed some time after sunrise upon the floor of Plato, partake of the 

 nature of clouds ? and are these clouds perforated and separated by elastic 

 vapours rising from a surface heated by an exposure to sunlight of 48 hours 

 or more, in consequence of which its reflective powers become stronger, pro- 



