PERIHELION MOTIONS. |21 



and part of May next following, although every part of the arrangement remained to all 

 appearance the same, yet the camphor was deposited on the side farthest from the sun. 

 From May until the present date, the deposite is again towards the sun. It does not 

 appear that any immediate cause can be assigned for this waywardness. Does it exist 

 in the sun's light? or in changes affecting the earth's atmosphere I or in imperceptible 

 changes in the instrument with which the observation is made? As respects the latter, I 

 think a negative answer may be given without any hesitation; but beyond a mere ex- 

 pression of the fact that these anomalous circumstances do occasionally occur, I would 

 not be understood to speak decisively ; if periodic changes like this do occur, which is 

 doubtful, they have not been watched for a sufficient length of time, nor have I made 

 sufficient variations in my trials to be able to refer them to any distinct cause. A large 

 bottle containing camphor, which has been deposited therein for more than a year under 

 ordinarv atmospheric pressures, has uniformly showed a crystallization towards the light. 



459. For making these experiments properly, it is necessary to possess an air-pump 

 receiver ground so true as to be able to maintain a vacuum for several hours, or even 

 da vs. A less perfect jar may be made to answer by fastening it down to the pump 

 plate with cement ; it will, however, be liable to leak when the cement becomes warm by 

 exposure to the sun. For many of these trials a barometer tube is sufficient. Those 

 who are provided with a good pump and jars, accompanied with their proper transfer 

 plates, will have no difficulty whatever. 



460. Upon the plate of the pump, or one of the transferors, a a {fig. 74), place some 

 camphor in a watch-glass, c, supported by a stand ; over this place a bell-jar, and ex- 

 haust until the difference of level of the siphon gauge amounts to half an inch or less 

 the farther the rarefaction is pushed the better then remove the arrangement into the 

 sunshine. In the course of five minutes, if the atmosphere be clear and the sun bright, 

 small crystalline specks will be found on the side nearest to the sun ; these continually 

 increase in size, and at the end of two hours many beautiful stellated crystals, from one 

 eighth to half an inch in diameter, will be found on that side, but on the other parts of 

 the glass only a few straggling ones here and there. This appearance is represented 

 mfis. 75 ; sometimes the whole side next the sun is covered with a deposite of camphor, 

 the other side containing none at all. 



461. Having made a torricellian vacuum, in a tube upward of 33 inches long and 

 five eighths wide, pass into it a piece of camphor, which will rise into the void. This 

 arrangement, like the former, when kept in the dark shows no crystallization, even 

 though so kept for more than four months; but on bringing the vacuum into a beam of 

 the sun. crvstallization rapidly goes on, and at the end of a quarter of an hour the ap- 

 pearance is such as is represented \nfig. 76. It is not important that the temperature 

 of the sunbeam or of the atmosphere should be high ; this is an experiment which 

 will succeed at temperatures varying from 120 Fah. to 60 Fah., and probably at much 

 lower degrees, for it is readily performed in the depth of winter. 



462. It is not a phenomenon connected with the process of crystallization. Take 

 ajar twelve inches high and four in diameter, quite clean and dry, place it over a glass 

 of water, b (Jig. 77), and expose it to the sunshine. In this experiment it is not re- 



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