i6o 



NATURE 



[December 14, 1893 



elevation, should be parallel to the contour. De Geer's map of 

 these isoanabatics, traced in 1890, satisfies these conditions, 

 allowance being made for the want of homogeneity in the rocky 

 mass, and the want of lixity of its borders. 



An interesting account of a fine series of glacial potholes on 

 Cooper's Island, Little Harbour, Cohasset, U.S., is given by 

 Mr. William O. Crosby, in a paper on the "Geology of the 

 Lujton Basin " (Occasional Papers of the Boston Society of 

 Natural History, IV.). It is shown that the potholes were 

 formed by vioulitis, or glacier mills, and Mr. Crosby discusses 

 a question raised in these columns a short time ago, viz. why, as 

 the ice-sheet moves continuously forward, carrying the crevasses 

 and mouliiis with it, the potholes escape elongation in the 

 direction of the movement ? The true explanation of many 

 glacial potholes is found in the fact, that a crevasse closes as it is 

 carried forward by the general movement of the ice, a new 

 one subsequently being formed just where in relation to the 

 land at the margin of the glacier the former one existed. This 

 explanation, however, is not applicable to the Cohasset pot- 

 holes, and in place of it Mr. Crosby makes the suggestion that 

 a moiilin may remain approximately stationary, while the ice 

 moves on, through the backward erosion and melting of its 

 up-stream side ; and that when a pothole is formed at the 

 bottom of a motdin, it is not the direct impact of the water upon 

 the face of the ledge that does the work, nor do the stones 

 carried down by the water wear the ledges appreciably by their 

 direct fall, but the pothole is due to their subsequent move- 

 ment, and especially their rotation, by the water. This rota- 

 tion implies an antecedent depression or hollow to hold the 

 stones, and thus the conditions are seen to be essentially the 

 same as for ordinary river potholes. Since the rotation of 

 stones in a pre-existing hollow appears to be an essential con- 

 dition of glacial as of other potholes, and the moulin simply 

 supplies the power, it would seem to make little or no dif- 

 ference whether the water plunges into the up-stream side, the 

 middle, or the down-stream side of the hollow. The pothole 

 is made where the hollow exist-, and during the progress of a 

 moiiIin across the hollow, there would not, apparently, be any 

 marked tendency to elongate it. In the case of a linear group 

 of potholes on the iceslape of a ledge, concludes Mr. Crosby, 

 it is reasonable to suppose that the upper one, which on 

 Cooper's Island is always the smallest and most indefinite, 

 marks the shifting position of the 7iioiilin, and that the others 

 were formed by the subglacial flow of water from the bottoin 

 of the jnoulin. 



It has been supposed, say? Mr. A. J. Jukes-Browne, in the 

 Geological Magazine for December, that the total amounts of 

 silica existing in the chalk with flints and the chalk without 

 flints respectively, are very nearly equal ; and this supposition 

 favours the theory that flints have been formed by some process 

 of segregation after the consolidation of the chalk containing 

 them. It is generally conceded that the silica from which such 

 flints were ma'ie was a soluble form like that of sponge spicules, 

 diatoms, or radiolaria ; hence by chemical analyses, aided by 

 microscopical discrimination between crystalline and colloid 

 siliceous particles, it is possible to determine whether flintless 

 chalk always contains soluble silica, and whether chalk with 

 flints contains little or none. Mr. Jukes-Browne has made this 

 investigation, and he finds that there is no definite relation 

 between the occurrence of flints and the absence or presence of 

 soluble silica in the surrounding chalk. He thinks that chalk 

 which is now destitute of any remains of siliceous spicules, has, 

 since it became chalk, always been destitute of such spicules. 

 These conclusions have a very important bearing upon the 

 question of the formation of flints. 



NO. 1259, VOL. 49] 



In a recent number of the Comptes Rendns, M. A. Dele- 

 becque gives the results of some observations made la^t summer 

 on water from various depths in inland lakes, which show that 

 the amount of solid matter in solution increases with the depth. 

 Thus in the lakes quoted below the amount of dissolved solid 

 matter in grammes per litre was : — Annecy, surface o'i38, 

 bottom (65 metres) 0'I57 ; Aiguebelette, surface o'li4, bottom 

 (71 metres) o'i6o5 ; Nantua, surface o'i54, bottom (43 metres) 

 o'igo ; Saint-Point, surface 0'I52, botto;ii (40 metres) o'i82 ; 

 Remoray, surface o'i6o5, bottom (27 metres) 0'205 ; Crozet, 

 surface 0'0275, bottom (37 metres) o'0368. The water samples 

 were collected about 3 metres above the bottom, by means of 

 Dr. H. R. Mill's water-bottle. M. Delebscque agrees with 

 Dr. Duparc, of Geneva, that the small amount of dissolved 

 matter in the surface water is due to its removal by the cal- 

 careous organisms which swarm in the upper layers. 



The Philosopliical Magazine for December contains a paper, 

 by Sidney J. Lochner, on the elongation produced in soft iron 

 by magnetisation. The author undertook the investigaion of 

 this subject in order, if possible, to settle whether the experi- 

 ments of Bidwell or Berget represented what really happens. 

 In order to measure the elongation, what was essentially a 

 Michelson's interferential refractometer was made use of, which 

 was capable of measuring an elongation of a millionth of an 

 inch. The bar of iron, whose elongation was to be measured, 

 was placed inside a long magnetising coil, and carried at one end 

 one of the mirrors of the refractometer. The expansion due to 

 the heating eflect of the coil being slow, while that due to 

 magnetisation was rapid, the two could be distinguished. The 

 author finds that, for a given magnetising field, different elonga- 

 tions are produced according to the manner in which the magne- 

 tising current is applied. Thus different elongations were pro- 

 duced in the cases where the current had been turned on suddenly, 

 or had been applied gradually ; and in the latter case it made a 

 difference whether the current had reached its final value by in- 

 creasing slowly, or by decreasing slowly from a higher value. 

 Another peculiarity observed was that if the current be gradually 

 increased from zero, at a certain point a maximum expansion is 

 reached; after this a further increase of the current will produce 

 a decrease in the elongation ; if, however, instead of increasing 

 the current when the maximum is reached, it is gradually de- 

 creased, it is possible to obtain a still greater elongation. The 

 observations show that the expansion is a function of the ratio 

 between the diameter and length of the bar, and that the 

 elongation varies approximately directly as the square root of 

 this ratio ; also, the expansion varies directly as the permeability. 

 The amount of current required to produce the maximum ex- 

 pansion also depends on the ratio between the diameter and 

 length. 



The bacterial efficiency of porous cylinders in the filtration 

 of water for domestic purposes is the subject of considerable dis- 

 cussion just now. Kirchner [Zeitschrift f. Hy^s^iene, vol. xiv, 

 p. 307) found in his expeiiments with water purposely infected 

 with typhoid bacilli, that such filters were incapable of arresting 

 these organisms. Large quantities of typhoid infected broth 

 were added to the water before filtration, and the filtrate after 48 

 hours was found to contain very large numbers of typhoid 

 bacilli. Dr. Schofer, in a recent number of the Centi-alblalt f. 

 Balderiologie, vol. xiv. p. 685, gives the results of his investiga- 

 tions of porous cylinders as regards their retention of typhoid 

 bacilli. In these experiments as small a quantity as possible of 

 nutritive material was added with the typhoid organisms to the 

 water (previously sterilised), and even after 24 days the filtrate 

 was found to be perfectly sterile, al; hough the unfi'.tered water 

 was freshly infected with typhoid bacilli no less than twelve 

 limes during the investigation. Very different results were, 



