126 APPLICATION OP PEINCIPLES. 



immediate cause of perspiration, which proceeds in 

 proportion to their intensity (71), yet this action is 

 necessarily modified by the state of the medium, that 

 is, of the atmosphere which surrounds them : in pro- 

 portion to its Wat and dryness will their power he 

 augmented, and h> proportion to its cold and mois- 

 ture diminished. The physiological effect of an 

 excessive augmentation of perspiration is to dry 

 up the juices and to destroy the texture of the leaves; 

 on the other hand excessive obstruction of that func- 

 tion prevents the decomposition and assimilation 

 of the fluids, and the foimation of new organised 

 matter, as well as of the secretions peculiar to a 

 species. A state of the atmosphere, therefore, which 

 is most favourable to the maintenance of the perspi- 

 ratory action in the most healthy state, is that which 

 it must be the business of a gardener to secure by all 

 the means in his power. 



Among the hygrometers intended for measuring 

 the quantity of elastic vapour in the atmosphere, the 

 most convenient for use is that invented by Professor 

 Daniell. In this instrument, the amount of moisture 

 in a given atmosphere is indicated by what is called 

 the dew-point; that is to say, by the point of the 

 thermometric scale at which the cold is sufficient 

 to cause a deposition of dew; the amount being cal- 

 culated by the difference between the natural tempe- 

 rature and an artificial temperature created for the 

 purpose of determining the point at which the elastic 

 vapour of the air is precipitated by cold. " The 

 natural scale of the hygrometer," says Mr. Daniell, 



