302 TEMPERATURE, ETC. [BOOK n. 



That the intensity of light does in fact vary most mate- 

 rially in different climates, is a matter of inference from the 

 difference of temperature. Sir John Herschel, in a communica- 

 tion to the AthencBum newspaper of April 25, 1835, gives the 

 force of sunshine at the Cape of Good Hope as 4 8' 7 5, while 

 ordinary good sunshine in England is only from 25 to 30. 



It is not, however, light alone which is capable of deter- 

 mining the quality of vegetable secretions; it is light in 

 connection with perspiration, and abundant food. The second 

 is promoted, and the latter augmented by continual currents 

 of air, which, rapidly passing over vegetable surfaces, abstract 

 water, and convey a ceaseless supply of carbonic acid for 

 absorption. Hence it is that plants in hothouses, although 

 exposed to the brightest light, are rarely comparable for 

 health or for the concentration of their secretions to the same 

 species in the open air. 



Temperature must also exercise a most energetic effect 

 upon those chemicovital actions which end in the production 

 of vegetable secretions. We are assured by Dr. Christison 

 that the (Enanthe crocata, one of the most venomous of our 

 wild herbs, as it grows wild in the neighbourhood of Edin- 

 burgh, is destitute of narcotic properties at all seasons. " The 

 juice of a whole pound of the tubers, the part which has 

 proved so deadly elsewhere, had no effect when secured in the 

 stomach of a small dog, either at the end of October, when 

 the tubers are plump and perfect, but the plant not above 

 ground, or in the month of June, when it was coming into 

 flower ; and an alcoholic extract of the leaves, and that pre- 

 pared from the ripe fruit, had no effect whatever when intro- 

 duced into the cellular tissue of the rabbit under the same 

 conditions in which the common hemlock acts so energetically. 

 By a comparative experiment he ascertained that tubers 

 collected near Liverpool, acted with considerable violence on 

 the dog." The same singular fact has been observed by Dr. 

 Christison in the Cicuta virosa, which he describes as being 

 innocuous in Scotland, or nearly so. In these cases it seems 

 difficult to explain the want of active properties, except upon 

 the supposition that the temperature of Scotland is too low 



