January 26, 1922] 



NATURE 



99 



has led to this interesting result. The tendency 

 among medical men was to regard every intrusive 

 Entozoon in the human body as '' a dangerous para- 

 site," and now we know not only that parasitic 

 Protozoa are not necessarily dangerous, but also 

 that many parasitic bacteria and even some worms 

 are not harmful to their hosts. 



Mr. Dobell is inclined to modify the ancient 

 and, as it seems to me, convenient use of the word 

 " parasite." It is not usual to regard every para- 

 site as " a dangerous parasite." One hears of a 

 " harmless parasite " also, and of " mere para- 

 sites." In fact, the Greek word means "alongside 

 the victuals," and signified in early times one who 

 had a seat at the table of sacrificial meats — an 

 officially established guest or messmate of the 

 priests. He w-as "venerable " rather than danger- 

 ous, and only when rich men took to entertaining 

 such pensioners for the purpose of display and self- 

 advertisement did " the parasite " fall into con- 

 tempt and was sneered at as a "toady." The use 

 of the word in zoology has been primarily in accord- 

 ance with this. The parasite of zoology infests or 

 hangs on to a host from which it obtains shelter 

 and food, but it does not necessarily injure its host. 

 There are many gradations between the harmless 

 necessary parasite and the deadly pest which con- 

 verts a "host" into a "victim" — absorbing its 

 life-blood or spreading deadly poison into its tissues. 



It is difficult to create a terminology which shall 

 in single words indicate the varieties of relationship 

 of parasite and host. The word " commensal " 

 was introduced by the elder Van Beneden. Ety- 

 mologically it has the same meaning as parasite, 

 but Van Beneden used it to signify specifically an 

 association in which the host suffered no injury or 

 inconvenience, but allowed a distinct species of 

 animal or plant to benefit by the scraps of food 

 rejected by itself, and even to get shelter and 

 carriage by its hospitality. At the same time. Van 

 Beneden pointed out that such close parasitism as 

 that of the intestinal worms is not necessarily in- 

 jurious to the animals infested, and he cited the fact 

 that, whilst in their normal wild condition the larger 

 carnivorous animals apparently without exception 

 harbour parasitic worms and are perfectly healthy, it 

 is found that the same animals in captivity tend 

 to lose their parasites. They, in fact, become un- 

 healthy and abnormal in captivity. The presence 

 in these animals of a few parasites is (according to 

 Van Beneden) normal and an indication of life in 

 health-giving conditions. For such reasons I should 

 prefer to retain the word "parasite" with its 

 original wide and general meaning, and to classify 

 by name (a somewhat troublesome task) the varieties 

 which it presents. E. Ray Lankester. 



NO. 2726, VOL. 109] 



Electric Furnaces. 



The Electric Furnace. By Dr. J. N. Pringf. (Mono- 

 g:raphs on Industrial Chemistry.) Pp. xii-f485 

 + 19 plates. (London : Longmans, Green and 

 Co., 1921.) 325. net. 



L TITLE more than 100 years have elapsed 

 since the first experiments on electric 

 furnaces vi'ere performed, when Davy, in 1810, 

 succeeded in isolating aluminium and the alkali 

 metals by the electrolysis of electrically fused 

 salts. Five years later Pepys carried out experi- 

 ments on the cementation of iron heated by pass- 

 ing an electric current through it. About half a 

 century elapsed, however, before a commercial 

 furnace was put into operation, one of the earliest 

 being built by the Cowles brothers at Milton, 

 Staffs. The development that has taken place 

 since that date may be gauged from the fact that 

 the estimated production of electric furnace steel 

 during 1918, in Great Britain alone, was 110,000 

 tons. This development was largely due to the 

 war and, as the last edition of Stansfield's classic 

 volume (bearing the same title as the book at 

 present under review) is dated 1914, a demand 

 doubtless exists for a further book on electric 

 furnaces. 



After dealing with the history and the principles 

 of electric furnaces, the author discusses the 

 types used in laboratory and experimental work. 

 He claims that it is possible to maintain a tem- 

 perature of 1200° C. inside a tube 8-6 cm. in- 

 ternal diameter, wound with No. I5 s.w.g. 

 nichrome wire. The makers of this material 

 state that it is suitable for use at temperatures 

 up to iioo°C., and while there is no doubt that 

 the higher temperature could be attained, the 

 author was indeed fortunate if his furnace did 

 not burn out in a short time. 



The next two sections of the book deal with 

 current supply and transformation in electric 

 furnace operations. These subjects are discussed 

 very fully, commencing with the theory of alter- 

 nating currents. There is a mistake in the first 

 figure in this section (Fig. 49), both collector 

 brushes of the diagrammatic dynamo being shown 

 on the one slip ring. 



A later section, 6, discusses the measurement 

 of high temperatures, dealing mainly with the 

 use of optical and radiation pyrometers. A 

 curious statement occurs on p. 84, where refer- 

 ence is made to thermo-couples consisting of 

 " two different metals such as platinum and an 

 alloy of platinum with rhodium or ruthenium." 

 Surely iridium, not ruthenium, is intended. 



The next three sections deal with the chemical, 



