Reviews — Prof. Duncan's Indian Geology. 419 



to separate them from the mixture of calcareous or organic matter 

 with which they are found united, it is usual to put the whole into 

 a glass test-tube with hydrochloric acid, adding caustic potash from 

 time to time, keeping all slowly dissolving by heat, in order to 

 isolate the silex, destroying the remainder. But in unbumt coal 

 it is too difficult to dislodge the carbon, and the acids have little 

 effect upon it. I must, however, refer to the calcination I effected 

 by grinding up the substance, and then, collecting it in a china 

 vessel, placed upon a stove in a glass tube, subjecting the whole to 

 the action of the heat, while, at the same time, a slight current of 

 oxygen crossing the tube combined with the carbon in creating 

 carbonic acid. 



Experience has taught me, however, the necessity of conducting 

 this operation at a lower temperature, in order to prevent the alkaline 

 or earthy bases and metallic oxides, which may be amongst the ashes, 

 from forming vitreous silicates by melting and mixing with the 

 valves of the Diatomacese. It is also well to leave the glass tube, 

 in which the fusing is going on, uncovered, in order to watch its 

 progress. The small residue obtained through this process is to be. 

 put into a clean test-tube, adding nitric acid and hydrochloric acid, 

 and caustic potash, assisted by the heat of a lamp to eliminate any 

 alkaline or earthy base, and every trace of metallic oxides. The 

 last operation over it only remains to wash repeatedly with distilled 

 water the very light dust which is left behind, letting it stand for 

 some hours each time to settle, in order to be sure of not losing the 

 smallest particle of it in pouring off the water. 



Those who follow this method exactly cannot fail to succeed. 

 The object may then be mounted with Canada Balsam, or in any 

 other suitable medium : and steadily and closely watching it under 

 the microscope, they will not be long before they see some valves of 

 Diatoms, entire or broken. 



If any investigator wish for fuller information, I shall have great 

 pleasure in gratifying him, and will consider myself honoured by his 

 applying to me. 



EEYIE "W S. 



I. — An Abstract of the Geology of India. By Prof. P. M 

 Duncan, F.K.S. (London, 1875.) 



THIS Abstract is a useful addition to geological literature. With- 

 out any pretension to a general treatise on the subject, and 

 consisting merely of geological facts, with no sections or illustrations, 

 it is intended as a text-book for the students of the Indian Civil En- 

 gineering College, where Geology is fortunately recognized as a 

 necessary part of their education, and as an advantage to their future 

 career. The late Mr. Greenough had collected a vast amount of ma- 

 terial, which he used in preparing his large geological map of India in 

 1854, a reduced copy of which, with notes, appeared in ' Petermann's 

 Geogr. Mittheilungen ' for 1855. But this knowledge has been 

 considerably increased by the subsequent labours of Carter, Drew, 



