406 Dr. Gotthelf Fischer's Essay on [Dec. 



the solar day has become longer. But M. Laplace has disco- 

 vered by theory that in the motion of the moon there is an 

 inequality known by the name of secular equation, which 

 depends on the variation of the eccentricity of the terrestrial 

 orbit, and the value of which in each century may be deduced 

 from the change of the eccentricity. By means of this equation 

 we give a complete account of the increase of velocity in ques- 

 tion. There is, therefore, no reason for supposing that the length 

 of the day has not remained constant. 



Let us admit for a moment, as Laplace does, that this duration 

 surpasses at present the hundredth of a decimal second, that of 

 the time of Hipparchus. The length of the century at present, 

 or of 36525 solar days, will then be longer than it was 2000 

 years ago (it is known that Hipparchus lived about 120 years 

 before our era) by 365-25"' decimal. In this interval of time, the 

 moon describes an arc of 534-6'' decimal. This quantity then 

 will express the difference between the arcs passed over by the 

 moon in a century at the present time and at the time of Hip- 

 parchus ; but as these arcs, determined by observations and 

 corrected for the secular equation, do not differ by so great a 

 quantity, we must conclude from it, that in this long interval, 

 the duration of the day has not varied by the hundredth part of 

 a decimal second. 



Article II. 



Essay on the Turquoise and the Calaite. By Dr. Gotthelf 

 Fischer, Professor of Natural History in the University of 

 Moscow. 



The term turquoise has been applied to two very different 

 -substances. The one, distinguished by the name of oriental 

 turquoise, is a true stone, a clay coloured by oxide of copper, or 

 even by arseniate of iron ; and belongs as much to the argilla- 

 ceous order of the oryctognostic system as chrysoprase belongs 

 to the siliceous order. I have placed it in the system under the 

 name of calaite, by which it had been already distinguished by 

 Pliny. The other substance, called simply turquoise, or oca- 

 dental turquoise, or turquoise odontolite, is a fossil, a petrefac- 

 tion, a tooth or a bone coloured by a metallic phosphate, which 

 does not belong to the mineral kingdom at all. Every part of 

 the skeleton may be in this way converted into turquoise, when 

 it happens to be placed in contact with coppery bodies, and 

 particularly with phosphate of copper ; but the fossil turquoise 

 capable of being employed in the arts is almost always a tooth, 

 which is harder than the other bones of the skeleton, and take* 



