26 



NATURE 



[May 9, 1872 



In these remarks I have limited my observations to those dol- 

 mens and covered chambers which are of the simplest grade, 

 but they would, I think, be equally applicable to the more com- 

 plicated structures similarly circumstanced. It seems, therefore, 

 premature to apply the term "dolmen mound" to such barrows 

 as Carnoct, for instance, where the small and accurately-made kist 

 bears no resemblance to the frenuent dolmens of the district. 



Harbottle, Rothbury, May 4 G. A. Lunouii 



The University of Freiberg 

 May I be allowed to ask in your pages for information about 

 the University of Freiberg in Saxony as a place of instruction in 

 Geology, Mineralogy, and Chemistry ? I should also be glad to 

 know which German Universities are held to be the best for 

 learning these tliree subjects, and when their terms end. Any 

 information on these points will oblige, 



Cambridge University, May 5 Undergradu,\te 



Sources of Sandstone 



Your geological readers will readily admit the importance of 

 any hint that will assist them in determining the source from 

 which a particular bed of sand or sandstone has been derived. 



How to discriminate between sand produced by the breaking- 

 up of quartz, and sand produced by the brcaking-up of flint, does 

 not appear to be generally understood. Chemical analysis gives 

 no assistance ; and when examined microscopically by ordinary 

 light, no difi'erence can be detected. 



Polarised light, however, differentiates these two substances in 

 such a decided way, that where they alone are concerned, no doubt 

 can remain for an instant as to whether a grain of sand consists 

 of one or the other. The quartr, as of course everybody knows, 

 is resplendent with prismatic colours, while the flint shows a cold 

 steel-grey surface covered with a peculiar marking, which I am 

 obliged to call a species of reticulation for want of a term more 

 exactly descriptive. This marking I consider to be indicative of 

 something in the structure of the organism replaced by the flint. 

 When once seen it is easily recognised. Some species of chert, 

 that from the Portlandian beds for instance, show the same mark- 

 ing as flint. 



I may also add that polarised light shows chalcedony to be, 

 like quartz, a crystalline instead of a non-ci-ystalline substance, 

 as usually taught. It bears the same relation to quartz crystals 

 and massive quartz, as fibrous gypsum does to selenite and com- 

 mon gypsum. . M. Hawkins Johnson 



Polarised Light 



We have all noticed that when the sun shines directly through 

 a window hung with figured muslin curtains, the reflection of the 

 pattern of the curtains m the window interferes with the prospect. 



When this reflected image is viewed through a Nicol's prism 

 it disappears when the prism'is rotated, leaving the prospect un- 

 obstructed ; the experiment; is very interesting, and can be per- 

 formed l)y any one » ho h.->s,a polariscope attached to a microscope, 

 and it is only necessary tp'observe that the image is viewed at the 

 proper angle. Th effect will possibly be best when the sun's 

 rays make an angleSJith the curtains and the glass nearly coin- 

 ciding with the polarising angle. (In my case the angle was 



SC 52'') 



Tyndall has mentioned a case in which the haze obstructing 

 the view of a mountain-top was rendered transparent by the Nicol. 



The readers of Nature have probably observed how com- 

 pletely the leaves of the ivy polarise light ; viewed through the 

 Nicol and a pink selenite, the plant appears covered with blossom. 



R. S. CULLEY 



CHOLERA AND SUN-SPOTS 



ON Friday evening, the 26th uU. Mr. B. G.Jenkins, 

 of the Inner Temple, read before the Historical 

 Society a remarkable paper on Cholera, founded on a 

 communication to the Russian Imperial Academy of 

 Sciences, and now under the consideration of the Medical 

 Council of the Minister of the Interior. Tlie author of 

 the piper maintained that no true advance could be made 

 in any science founded on experience, and looking to facts 

 for its development, until the history of that science had 

 been recorded and correctly mterpreted ; and that it was, 

 because we have been looking at the facts of cholera, 



which have been accumulating for half a century, as facts 

 without attempting to show, or rather without succeeding 

 in showing, in what relation they stand to each other, 

 that we are really no w'iser than we were forty years ago. 



He held that, instead of one "home" of the cholera 

 in the delta of the Ganges, there are seven, all situated 

 on or near the Tropic of Cancer, equally distant from 

 each other, of which the most important is that at the ^ J 

 mouth of the Ganges ; the others are to the east of 

 China, to the north of Mecca, on the west coast of Africa, 

 to the north of the West India Islands, to the west of : 



Lower California, and among the Sandwich Islands ; that ' 



a reference to the map would show that the recorded ap- 

 pearances of cholera over the globe may be satisfactorily 

 explained by supposing seven atmospheric streams, each 

 1,400 miles in breadth, to proceed from these foci in a 

 north-westerly direction ; and that at some periods, as 

 1 833, 1 850, and 1 866, nearly all the streams were in activity. 



Having pointed out the course of these streams on a 

 map especially prepared, and shown how the disease 

 moved within the limits of each, in both the north-west 

 course and its south-east extension across the Equator, 

 the author, in tracing in detail the course of the cholera 

 in India during iSi 7 and 1818, called attention to a re- 

 tiiarkable law which nianifested itself, a law which he 

 held was generally applicable wherever cholera appeared. 



Although the course of cholera during 1817 was not 

 very clear, still it is evident that it was north-west and 

 south-west. The lull in virulence and advance which 

 occurred in December 1817 continued to March 1818, 

 when cholera broke out again just where it had ceased 

 the previous December. He drew attention to a very 

 recent similar instance ; the cholera last year halted on 

 the western border of Russia, and about a fortnight ago 

 broke out in Poland, which augurs ill for the North of 

 England this year. The remarkable law which the author 

 pointed out was that in 1818 the cholera advanced simul- 

 taneously in two directions, north-west and south-west, 

 in such a manner that all the places attacked at any 

 given time by its north-west advance were situated at 

 right angles to all the places attacked at the same time 

 by its south-west advance. This double advance is made 

 evident by cutting a piece of paper square, placing a 

 corner upon the map at Calcutta, and moving it across 

 India in a direct line to Surat. In iSig the cholera crossed 

 the Arabian Sea to Muscat, and passed simultaneously 

 through Persia, and up to 1S23 advanced as far as Asia 

 Minor and the Caspian, and then ditd out. In 1823 a 

 fresh outbreak occurred in India ; this sttadily proceeded 

 to the north-west, and halted in the west provinces of 

 Russia in 1 830, and the next year broke out in full force 

 in the same locality, thus presenting a parallel to 187 1-2, 

 and went as far as Britain. By referring to the map it 

 will be seen that all places attacked by this stream of 

 cholera in 1S31 lie within the boundaries represented by 

 [ two lines, one drawn from the southern point of India to 

 the north of England, and the other from the Ganges 

 through Orenburg to Archangel. The author having de- 

 scribed with great minuteness the rise and progress of 

 the other six streams, bringing the subject down to the 

 present day, stated that Europe was liable to attacks from 

 two great sources, India and Arabia: Russia and Northern 

 and part of Central Europe coming under the influence 

 of the Indian stream ; Southern and Western and part 

 of Central Europe under the influence of the Arabian ; 

 and that the Continent would certainly be attacked by 

 both this year. The curious cases of ships at sea being 

 suddenly attacked by cholera, and, again, the instances of 

 ships sailing along the coast of India being struck by the 

 disease when at the same place, he explained on the sup- 

 position that they had been sailing within the limits of 

 the cholera streams ; for when they got outside the limits 

 the disease suddenly ceased. He called attention to a 

 fact worthy of mention, that all the places recorded by 

 Dr. Gavin Milroy as unaffected hitherto by cholera lie 



