Fkbrdauy, 1902.] 



KNOWLEDGE 



29 



Mr. A. R. Wallace iu Tropical Xatiire reiers to the 

 stone mesemlirydnthemum at the Cape described by 

 Dr. Biirchell, wiiich closely resembles in form and colour 

 the stones among which it grows; on this account, its 

 discoverer believes the juicy little plant generally 

 escapes the notice of cattle and wild herbivorous 

 animals. Mr. J. P. Manscl Weale mentions that in 

 Kaioo mauv plants have tuberous roots above the soil 

 resembling stones so perfectly that it is almost impossible 

 to distinguish them. Tlie tubers of the potato itself in 

 its native home may perhaps be protected in this w'ay. 



The last'Hientioned observer has also noted a labiate 

 plant, Ajutja ophn/dln, in South Africa which bears a 

 strong resemblance to an orchid. As this is the only 

 species of bugle in the district Mr. Wallace thinks tho 

 flower profits by the mimicry and succeeds in attracting 

 the insects required for its fertilisation. A species of 

 balsam at the Cape has also acquired an orchid-like 

 aspect. At least one phanerogam bears a lichen-like 

 aspect; TiU(in<hi(t V^nenidex, one of the pine-apple 

 family, grows on trees in tropical America and has a 

 resemblance to a shaggy lichen so marked that ib is 

 generally mistAken for a plant of that order. The fly 

 agaric, our most conspicuously coloured fungus, accord- 

 ing to Dr. Plowright, is closely imitated by a parasitic 

 flowering plant Balanophora vnlucrata, the scarlet cap, 

 the dotted warts, the white stem and volva being all 

 accurately represented. 



THE HISTORY OF FAHRENHEIT'S 

 THERMOMETER. 



By Sir Samuel Wilks, m.d., i.l.d., f.e.s. 

 The origin of Fahrenheit's thermometer and the meaning 

 of the scale are not to be found in any work on natural 

 philosophy or chemistry with which I am acquaiuted. aud 

 all the professors whom I have met have expressed like 

 ignorance. Although the instrument aj>pears inexplicable 

 and unwieldy, there is great interest attached to its 

 history, when we learn that it was really invented l)y Sir 

 Isaac Newton, and that the starting point of his scale was 

 the heat of the human body. 



Newtou's paper is to be found in the Philoaophical 

 Transactions for the year 1701. He describes his instru- 

 ment as a glass tube filled with linseed oil, aud to it he 

 attached a scale to measure the degree of heat of the 

 li(|uid into which he plunged it. His lowest point was 

 that of freezing, as his highest was that of boiling water. 

 He chose for the starting point on his scale the heat of the 

 human body, and this he called by the round number \2. 

 the iluodecimal system being then in use ; that is, he 

 divided the space between the freezing point and the 

 temperature of the body iuto twelve parts. He further 

 stated that the boiling point would be about 30, as it was 

 nearly three times that of the human body (see Fig. 1). 



A few years afterwards, when Fahrenheit was workin" 

 at the subject of heat, he took Newton's instruments for 

 his experiments, l)ut, finding the scale not minute enouLrh, 

 he divided each degree into two parts, and so made it 

 measure 24 instead of 12. He also did more, for finding 

 he could obtain lowc temperatures than freezing, and 

 notably that of ice and salt mixed together, he took this 

 for his starting point. It was from this point he bescan 

 to count 24 degrees up to body heat. This made, by his 

 measurements, 8 the point for freezing. Boiling point he 

 made 53. It then became zero, freezing 8, bodv heat 24, 

 and boiling water 53. This was really the same as 

 Newton's, only the scale started lower and the numbers 

 were doubled (see Fig. 2). 



Later on, finding that he could measure increments of 

 heat more minutely, Fahrenheit divided each degree into 

 four parts. It will now be seen that if the numbers just 



NEWTON 5 



Boihng 30°(about) 



-- 12. " Body 



3- Freezing 



Fahrenheits 



Boiling Si" 



24° Body 



8° Freezing 

 ■Zero 



Boili.ng CI2* 



-36° Body 



-32° Freezing 

 Zero 



Fig I Fig 2 FiG 3 



mentioned are multiplied by four we have the thermometer 

 which is now in use. Beginning with zero, freezing 

 becomes 32, the body heat 96, and boiling point 212. 

 This is the scale which Fahrenheit made, and the reasons 

 for his doing so. It is the one which has been in use 

 ever since {see Fig 3). 



Of late years, when more accurate readings were required, 

 each degree was divided into ten. This brings the decimal 

 system into a scale which was before wholly duodecimal. 

 It may be stated that Newton was aware that blood heat 

 was higher than that which he obtained by placing the 

 thermometer under the arm to measure the body heat. 



SILK AND ITS PRODUCERS. 



By R. Lydekker. 



Whereas wool, fur, and hair are exclusively the pro- 

 ducts of a single class of vertebrato animals, namely, 

 the mammalia, silk is as exclusively a product of in- 

 vertebrate creatures. In place, however, of being pro- 

 duced by one single class, it is yielded in a workable 

 foiTU by certain membei-s of at least three distinct 

 classes of the lower animals, namely, the lepidopterous 

 insects, the spiders, and the bivalve molluscs. A<> 

 present, however, it is only the product of the cater- 

 pillars of a comparatively few species of moths that is 

 of any real commercial importance. 



Unfortunately the manufacture of raw silk into the 

 finished article is one of those industries which have 

 to a gi-eat extent passed out of the hands of the British 

 workman into those of his foreign rivals ; and the 

 Spitalfields hand-loom weaver is almost as extinct as 

 the dodo, while the liband-mills of Coventi-y have 

 largely been superseded by cycle-manufactories. 



According to an article which appeared a few years 

 ago in Ciinimerrial Intelligence, it seems that during 

 1898 the imports into Great Britain of manufactured 

 silken goods amounted to over twenty millions in value, 

 while the imports of raw silk were only about one million. 

 Inclusive of the necessary labour, the value of this 



