August 25, 1923] 



NA TURE 



293 



Research Items. 



Tests of Natural and Culture Pearls. — A 

 simple optical method of distinguishing the Japanese 

 " culture " pearls from wholly natural pearls is de- 

 scribed by Dr. F. E. Wright in Joum. Washington 

 Acad. Sci., 1923, vol. 13, p. 282. In a bead of 

 mother-of-pearl, such as is always used for the nucleus 

 of the " culture " pearls, the nacreous layers are not 

 concentric to the surface, but are approximately plane, 

 being parallel to the surface of the shell from which 

 the bead was cut. Now normal to this surface the 

 reflecting power, and consequently also the opacity, 

 is at a maximum ; whilst at 90° from this direction 

 (that is, looking along the laminae) there is a minimum 

 of reflection and of opacity. A " culture " pearl when 

 viewed in a strong reflected light (for example, with 

 the observer's back to the sun) shows at the opposite 

 poles of one diameter a small bright spot due to the 

 light reflected from the laminae of the enclosed bead 

 of mother-of-pearl. In a strong beam of transmitted 

 light (arranged in a closed box with lens and mirror, 

 the pearl resting in a circular aperture) the " culture " 

 pearl shows two positions of maximum opacity, whilst 

 the natural pearl is the same in all positions. A third 

 method, which is applicable also to " culture " pearls 

 containing a real pearl as nucleus, is given by an 

 examination of the walls of the hole drilled through 

 the pearl. The pearl is illuminated by a strong side 

 light and a minute bead melted on the end of a gold 

 wire is inserted in the bore to act as a reflector, 

 which is viewed under the microscope. 



Condensed Milk. — An important report by Dr. 

 Savage and Mr. Hunwicke on the manufacture, con- 

 dition, bacteriology, and spoiling of commercial 

 sweetened and unsweetened condensed milk has been 

 issued by the Food Investigation Board (Special Rep. 

 No. 13). The changes in the condition of the milk 

 as a result of its concentration are profound, and not 

 merely those caused by deprivation of water. It is, 

 for example, a much worse conductor of heat than 

 unconcentrated milk. While sporing aerobic bacilli 

 are present in a considerable proportion of samples, 

 decomposition and spoiling are nearly always due to 

 non-sporing bacteria, particularly certain micrococci, 

 which either survive the preliminary pasteurisation of 

 the raw milk in the course of manufacture, or after 

 canning are admitted to the tins through minute leaks. 

 The sources of bacterial contamination and multipli- 

 cation are mainly from the original milk, from the air 

 of the factory, and particularly from dirty pipes and 

 apparatus. As regards the viability of the micrococci 

 which cause spoiling, in unsweetened condensed milk 

 they may survive a temperature of 70° C, but are 

 destroyed at 80° C. in a short time : this suggests that 

 a longer pasteurisation of the raw milk might be an 

 advantage. The best manufacturers appear to have 

 achieved striking success, however, in dealing with 

 such an unstable substance as milk. 



Researches on Marine Animals. — We note with 

 pleasure that Prof. M'Intosh — the veteran naturalist 

 — continues to publish his notes from the Gatty 

 Marine Laboratory, the forty-fifth paper of this series 

 appearing in the July number of the Annals and Maga- 

 zine of Natural History. A note on variation in the 

 wild rabbit is included, but the other items refer to 

 marine animals. The results of a comparative study 

 of the British species of Lepadogaster (Sucker-fishes) 

 are set forth, the characters of the young as well as 

 of the adults being contrasted. The sub-fossil skull 

 of a whale found at Airthrey, near Stirling, is de- 

 scribed and figured, and Sir William Turner's con- 

 clusion that it pertains to Sibbald's Rorqual is 

 corroborated. A fragmentary skull of Balcsna 



NO. 2808, VOL. I 12] 



australis, from the Campbell Islands, is also described. 

 Finally the variation of Amphinome rostrata, a 

 Polychaete worm, is considered, and the conclusion is 

 arrived at that the differences said to exist between 

 specimens from the Atlantic and Indian Oceans are 

 not specific, but are largely due to different methods 

 of preservation. Formalin is condemned as an un- 

 suitable preservative for animals of this group. 



Virus Diseases of Potatoes. — A valuable addi- 

 tion to knowledge of the virus diseases of the potato 

 (Leaf Roll, Mosaic, etc.) has been made by P. A. 

 Murphy, of Dublin, who publishes an account of his 

 work in the current issue of the Journal of the Irish 

 Department of Agriculture. It is now well established 

 that what has hitherto been called degeneration of 

 the potato is not due so much to environmental causes 

 as to the presence of infective diseases of which the 

 perplexing feature is that no visible causative organ- 

 isms have yet been discovered. The menace of these 

 virus diseases to the economic plants of the world 

 seems to be increasing. Already the maize and sugar- 

 cane crops in America have been attacked over large 

 areas of the country. In addition to the potato 

 these diseases (it may be the same disease) in the 

 British Isles attack the tomato, mangel, and the hop. 

 It was shown some years ago by Quanjer in Holland 

 that the infection is transmitted from plant to plant 

 by species of aphis. Mr. Murphy has now proved 

 that other insects infesting the potato, such as 

 various species of Jassid and Capsid, are capable 

 of transmitting infection. In this connexion it is 

 interesting to learn that potatoes when grown in the 

 North of Scotland are not so liable to infection, and 

 it has been suggested that this is due to the absence 

 of disease-carrying insects at a sufficiently early stage 

 of the growth of the plant. Whatever the reason 

 may be, it is undoubted that tubers imported into 

 the south from this region are generally free from 

 disease and produce a much heavier crop of potatoes 

 than that raised from indigenous " seed." Mr. 

 Murphy also shows that certain varieties of potatoes 

 are less liable to infection than others. One of his 

 most interesting experiments was an attempt to secure 

 healthy tubers for " seed " by " rogueing " out ob- 

 viously infected plants. The result was unfortunately 

 inconclusive, and it appears doubtfxil whether im- 

 munity from attack can be secured by this means. 

 Another perplexing feature of these diseases probably 

 stands in the way. There appears to be no doubt 

 that certain plants (including Solanaceous weeds) act 

 as carriers of the disease, and may therefore be the 

 means of infecting other plants, while showing no 

 visible signs of infection themselves. 



Termites of Barkuda Island. — In a recent part 

 of the Records of the Indian Museum (vol. xxv, 

 part II.) is a memoir on the Termites of Barkuda 

 Island in the Chilka Lake. The systematic char- 

 acters of the genera and species are described by 

 Prof. F. Silvestri, the habits by Dr. N. Annandale, 

 and the fungi cultivated by the termites by Prof. 

 S. R. Bose. Dr. Annandale divides these termites 

 biologically into three categories — burrowers, mound- 

 builders, and log-dwellers — a classification which, as 

 he points out, does not correspond with the taxonomic 

 one. He discusses the swarming, the duration of 

 life, the cultivation of fungi and the search for food, 

 and the details of structure of the nests. 



Parasitic Nematodes. — Dr. H. A. Baylis and 

 Mr. R. Daubeny {Memoirs Indian Mus., vii. pp. 263- 

 347) report on the parasitic nematodes in the collec- 

 tion of the Zoological Survey of India. The material, 

 which includes about eighty species, was collected 



