Notices respecting New Books. 301 



a most important physical change commenced in the comet's state. 

 Previous to that instant it must have consisted of a mere nucleus, a 

 stellar point more or less bright, and a coma more or less dense and 

 extensive. At that instant the formation of the envelope commenced, 

 and continued in the manner and at the rate above described. The 

 Ray, or internal comet, no doubt, also commenced its existence at 

 the same instant." This result was corroborated by an observation 

 mavle at Breslau by Prof. Boguslawski, who on the 22nd of January 

 observed the comet, as a star of the 6th magnitude, a bright concen- 

 trated point which showed no disc with a magnifying power of 140. 



Sir John next proceeds to examine the hypotheses respecting the 

 physical constitution of the comet which the phsenomena render 

 probable. He observes, " It will not be necessary to enter into any 

 calculation of the comet's distance from the sun at the moment of 

 these observations to warrant our rejection of an hypothesis such as 

 that of M. Valz, which makes the volume depend directly on the 

 distance. Undoubtedly its recess from the sun is ultimately, but 

 indirectly, the cause of the observed increase. The question is only 

 as to the modus operandi. The perihelion passage, it will be borne 

 in mind, took place on the 15th of November, and it was not there- 

 fore till the 83rd day after that event that the formation of the enve- 

 lope commenced. Now during these 83 days it is certain that a 



process of refrigeration had been going on The surface of the 



nucleus must be supposed to have cooled down, at this precise epoch, 

 to the dew-point. . . . At this instant, therefore, must have commenced 

 the formation of a mist, first precisely in contact with the radiating 

 surface, and by degrees more or less rapid ascending above it, and limited 

 by a definite upper surface, the seat of a fresh process of radiation. 

 As this superior vapour plane attains a higher and higher level, corre- 

 sponding to a less and less actual density both of the atmosphere and 

 vapour, its surface, without losing its geometrical form, necessarily 

 becomes less and less visible and conspicuous It is this su- 

 perior vapour plane which constitutes, in our conception of the mat- 

 ter, the visible surface of the envelope." 



In reference to the form of the envelope, he says, " If the conden- 

 sible vapour were everywhere distributed spherically around the 

 nucleus, so as to have equal density at equal distances, the form of 

 the envelope would necessarily be spherical, because on account of 

 the exceeding tenuity of the cometic atmosphere, its posterior por- 

 tion is in no way shaded by its anterior, but every part, interior and 

 exterior, equally exposed to the sun's rays, or very nearly so. That 

 the form is in fact not spherical, but paraboloidal, or very eccentrically 

 elliptic, instructs us in an important and truly wonderful fact, viz. 

 that the surfaces of equilibrium of the vapour in its transparent state 

 are so ; that, in fact, although not seen the envelope existed as a trans- 

 parent atmosphere — a tail in posse, though not to our eyes in esse, 

 and of its full extent, prior to the epoch of its commencing visibility, 

 and probably prior to the parhelion passage." 



He next proceeds to inquire by what forces it is possible that such a 

 form of equilibrium can be maintained, and remarks that the laws of 



