Things not generally Known. 



tide, all that we know with certainty is, that the resultant 

 effect of all the thermal agencies to which the earth is exposed 

 has undergone no perceptible change within the historic period. 

 We owe this fine deduction to Arago. In order that the date 

 palm should ripen its fruit, the mean temperature of the place 

 must exceed 70deg. Fahr. ; and, on the other hand, the vine 

 cannot be cultivated successfully when the temperature is 

 72 deg. or upwards. Hence the mean temperature of any 

 place at which these two plants flourished and bore fruit must 

 lie between these narrow limits, i. e. could not differ from 

 71 deg. Fahr. by more than a single degree. Now from the 

 Bible we learn that both plants were simultaneously cultivated 

 in the central valleys of Palestine in the time of Moses ; and 

 its then temperature is thus definitively determined. It is the 

 same at the present time ; so that the mean temperature of 

 this portion of the globe has not sensibly altered in the course 

 of thirty-three centuries. 



THEORY OF CRYSTALLISATION. 



Professor Pliicker has ascertained that certain crystals, in 

 particular the cyanite, " point very well to the north by the 

 magnetic power of the earth only. It is a true compass-needle ; 

 and more than that, you may obtain its declination." Upon 

 this Mr. Hunt remarks : " We must remember that this crystal, 

 the cyanite, is a compound of silica and alumina only. This 

 is the amount of experimental evidence which science has 

 afforded in explanation of the conditions under which nature 

 pursues her wondrous work of crystal formation. We see just 

 sufficient of the operation to be convinced that the luminous 

 star which shines in the brightness of heaven, and the cavern- 

 secreted gem, are equally the result of forces which are known 

 to us in only a few of their modifications." Poetry of Science. 



Gay Lussac first made the remark, that a crystal of potash- 

 alum, transferred to a solution of ammonia-alum, continued to 

 increase without its form being modified, and might thus be 

 covered with alternate layers of the two alums, preserving its 

 regularity and proper crystalline figure. M. Beudant after- 

 wards observed that other bodies, such as the sulphates of iron 

 and copper, might present themselves in crystals of the same 

 form and angles, although the form was not a simple one, like 

 that of alum. But M. Mitscherlich first recognised this cor- 

 respondence in a sufficient number of cases to prove that it was 

 a general consequence of similarity of composition in different 

 bodies. Graham's Elements of Che, 



IMMENSE CRYSTALS. 



Crystals are found in the most microscopic character, and 



