204 TEANSACTIONS OF THE 



given conditions carbonic acid absorbs 972 times, and ammonia 

 5460 times as much heat as air or oxygen. Similar experiments 

 with perfumes show that these possess the power of arresting the 

 passage of calorific rays to a remarkable extent. Thus, air scented 

 with thyme arrested 33 times, with peppermint 34, spearmint 38, 

 lavender 32, wormwood 41, cinnamon 53 times as much heat as 

 air did. The temperature of dry air is not affected by the passage 

 of the sun's rays through it. It is transparent and diathermous. 

 The capacity of the atmosphere for heat is therefore in proportion 

 to the aqueous vapour it contains. In a region subjected to the 

 rays of a hot sun through the day, the heated soil would at night 

 speedily cool down by rapid radiation through the dry atmosphere, 

 which would not itself be sensibly affected by this heat, the 

 result being a great fall of temperature, such as would doubtless 

 be injurious to'plant life. The perfume shed by aromatic glands 

 may, by supplying the deficiency of aqueous vapour, present a 

 curtain, so to speak, which would obviate the injurious loss of 

 heat, on the principle that a clear night is colder than a cloudy 

 one. As a matter of fact, we know that the flow of sap from the 

 Maple is much larger on a cloudy evening than on a clear one, 

 and the flow of sap is an important factor in vegetable growth. 

 Confirmatory too, of this view, aromatic plants are most odori- 

 ferous at night or before dawn, the coldest period. Such, then, 

 would appear to be the original intention of odours like that of 

 the Sweet-brier, and we have already seen the probability that its 

 thorns should be traced to a similar origin, viz., a dry hot climate 

 and poor stony soil. 



V. This class should perhaps have been included in the second, 

 as the epidermal scales on shore plants, (Chenopodium, Eringo, 

 Silene, etc.) are the representatives of hairs, on the sea margin 

 hairs being objectionable, on account of their collecting salt from 

 the sea water ; scales, scurf, or bloom, serving to run it off, as the 

 oil on the feathers of aquatic birds does. 



From these and similar reasons we have been led to ascribe the 

 remarkable liking for human abodes which many species of plants 

 possess, to man's influence on the soil and climate, who, by cutting 

 woods, draining, etc., renders these drier and so more suited to 

 members of a flora — settlers which came originally from a less 

 moist country than our own, and which may possibly have formed 



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