530 



NATURE 



[Oct. 2, 1879 



exhibited the poor animal dead. If it had been struck by the 

 snake, 10-15 minutes would have elapsed before death. I 

 still seemed convinced, and on his coolly asking for the 

 chicken for his dinner, I said I could not think of allowing him 

 to eat a poisoned animal, and so ordered it to be buried. 



Having received their bakhsheesh, both men asked me for 

 some brandy : as, at the moment, there was none in the house, 

 and telling them so, one pointed to a large bottle of saturated 

 tincture of ginger which was standing in the sun, and asked what 

 it was. On my telling him, both asked for some, so I bade them 

 sit down, and poured a mouthful down each gullet. The unex- 

 pected pungency of the shrub astonished them, but one of them, 

 pressing his stomach with both hands, and with his eyes streaming 

 with tears, gasped out "aur do " (more give). The other man 

 said he had had enough. Both then rose, and shouldering their 

 baskets, salaamed and left the compound. 



Skin skidding at?iong Snakes. — Though I have handled exuviae 

 by the hundred, and some of them just cast, I have never 

 witnessed the process of skin-shedding, nor, I believe, has any 

 observer. 



It is well known that the skin is always found inverted, and 

 very often, quite entire ; and the general impression is that the 

 snake fixes itself in a bush, or strong grass, and then wriggles 

 out of its skin. But I have found the skin on the floor of a 

 bath room, and on the rough ballast of a railway. 



I believe that fixture is obtained by means of the abdominal 

 scales, and that then the modus operandi is as follows : the 

 skin ready to be cast, yields round the snake's mouth only, 

 and remains adherent to the extremity of the tail. As the animal 

 advances the caudal extremity of the skin is inverted, that is, 

 pulled inwards, and so the process goes on, and is completed by 

 the tail passing through the mouth of the skin ; and thus the 

 direction of the abandoned skin is directly opposite to the 

 direction taken by the skin-casting snake. That is, if the mouth 

 of the skin lies east, the snake went out to the west. 



Take the finger of a glove, and pass a knotted thread inwards 

 through its tip, then pull gently on it, and the tip of the glove 

 will pass inwards and downwards, and ultimately pass through 

 the base of the finger, which will now be uppermost. 



Peshawar H. F. Hutchinson 



( To be continued, ) 



A Plague of Rats 



I HAVE read with great interest in Nature, vol. xx. p. 65, 

 a note of Mr. OrviUe A. Derby's on plagues of rats in Brazil. 

 The same thing occurs sometimes in the south of Chile, 

 Araucania, Valdivia, and Llanquihue, when the Coligue, 

 and other species of the Bambuseo; have flourished and 

 fructified, an occurrence which happens every 13-25 years. 

 These grasses, with solid canes, unbranched, of sometimes more 

 than 10 metres long and 8 cm. thick, flourish only once in their 

 life, when they are 15-25 J'ears old, and then their fruits ripen 

 in astonishing quantity. This causes an enormous multiplication 

 of rats and mice in the woods, animals rather rare commonly ; 

 and at the end of the same or the beginning of the next year, 

 these animals have finished wilh their food, and are then obliged 

 to migrate to the cultivated district-, where they are very 

 noxious. The Indians collect the seeds of the Coligue as food, 

 as the Brazilian natives seem to do with the fruit of the bamboo. 

 I had occasion to observe this fact in 1S69 or 1870, when I lived 

 in Valdivia, and when almost all the Coligues of the province 

 flourished at once and died afterwards ; and I had heard it 

 already before from the natives. Federico Fhilippi 



Santiago, Chile, August 17 



Solar Halo 



On Monday, September 22, about 12 o'clock, on the coa't at 

 Burnham, Somerset, my little boy called my attention to a large, 

 clearly-defined, white circle, of which the zenith might be the 

 centre ; on the southern side of the circumference was the sun, 

 above which were the arcs of two other circles, one of which 

 ■was flattened. They united at a small distance above the sun, 

 and displayed rather dull prismatic colours ; between the points 

 where these arcs joined the large white circle were two rather 

 oval-shaped patches, also showing prismatic colours. The 

 appearance lasted about an hour and a half. G. MapletON 



Badgworth Rectory, Weston-super-Mare, September 23 



CHEMICAL ACTION 

 VyHY are the properties of bodies so profoundly 

 ' • modified by the action called chemical ? Why do 

 certain bodies otily act chemically upon one another? 

 What exact meaning is to be attached to the expression 

 "chemical affinity?" 



These questions, and questions such as these, have 

 engaged the attention of chemists since chemistry began 

 to be an exact science. 



The products of chemical action are innumerable : 

 chemical science is encumbered with a multitude of 

 compounds, and each day additions are made to the 

 number; but no general theory of chemical action has 

 yet been broached which suffices to explain the known 

 facts. 



The consideration of the initial and final distribution 

 of matter in a system upon which chemical action is 

 exerted, has almost entirely engaged the attention of 

 chemists, to the exclusion of the study of the course of 

 chemical change, the conditions modifying this change, 

 and the nature of the force which causes the change. 



The molecular theory of matter furnishes us with a 

 fairly complete answer to the question — Wherein consists 

 the essential characteristic of chemical action ? 



Chemical action, says this theory, results in the pro- 

 duction of new molecules, mechanical action results in 

 changes in the rate of motion of existing molecules. 



But why are new molecules formed only when certz.in 

 bodies are brought into contact and not when other bodes 

 are placed under similar conditions ? 



Because the first substances exert chemical affinity upon 

 one another, whilst the others do not. 

 But what is chemical affinity ? 



The expression affinity was originally used to denote a 

 resemblance between certain substances which exerted an 

 action of some kind upon one another. But when the 

 study of chemistry advanced, it was found that those bodies 

 which most readily exerted mutual chemical action, were, 

 as a rule, unlike in their chemical habitudes. 



The expression affinity was, however, retained to 

 express the fact that one body exerted chemical action 

 upon another. This affinity could not be measured in 

 terms of any unit, hence chemists M-ere content to draw 

 up tables of relative affinities. These tables were for the 

 most part based upon qualitative reactions, and supplied 

 merely empirical information. 



In the year 1780 Bergmann formulated a general theory 

 of chemical affinity : the main points insisted upon by 

 Bergmann were, that the affinity between two bodies is 

 independent of the masses of the bodies brought into 

 mutual contact, and that the value of this affinity is con- 

 stant under similar conditions. Bergmann further supposed 

 that the relative affinity values of various substances may 

 be empirically represented by the amounts of these bodies 

 which mutually combine together : thus in the formation 

 of a series of normal salts, the affinity of the acid is 

 greatest according to Bergmann, for that base, the 

 greatest amount of which is taken up by the acid. Con- 

 versely a base has the greatest affinity for that acid which 

 combines with it in greatest quantity. 



The latter part of Bergmann's theory could no longer 

 be upheld when the atomic theory of Dalton had intro- 

 duced clearer views concerning the quantitative action of 

 chemical substances upon one another. But the atomic 

 theory was not opposed to the view that the affinity 

 between the bodies is independent of the masses of the 

 bodies brought into mutual contact. 



In the year 1803 Berthollet published his theory of 

 chemical affinity, a theory which was essentially opposed 

 to that of Bergmann. The French chemist said that the 

 chemical action of one substance upon another is propor- 

 tional to the mass of the acting body and to its affinity for 

 the second substance. Berthollet thus considered not only 

 the affinity of one body for another, but also the masses 



