178 THE HTGEOMETEK. 



the Becretions peculiar to a species. A state of the atmosphere, 

 therefore, which is most favourable to the maintenance of the 

 perspiratory 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. 



The fitness of an atmosphere for maintaining a healthy 

 vegetation depends upon the amount of moisture suspended in 

 it, and upon its temperature. The hygrometer indicates the 

 former, as the thermometer does the latter. 



Among the hygrometers intended for measuring the quantity of 

 elastic vapour in the atmosphere, the most convenient for use is that 

 invented by 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. 



It is impossible for any one to know what degree of moisture 

 he really maiatains in a forcing-house without an instrument by 

 which to measure it ; that instrument is the hygrometer. Of Daniell's 

 hygrometer the annexed cut exhibits the general appearance. It 

 measures the moisture in the air quickly and precisely, and is 

 not subject to get out of order. The air we breathe is a per- 

 manently elastic fluid, containing watery vapour in mixture, its 

 power of retaining which is greater when temperature is high than 

 when low. It may be compared to a sponge ; if this substance, when 

 dry, is soaked in water, a portion of the fluid is absorbed ; but if the 

 sponge is again dipped without squeezing, and before it has had time to 

 dry, no additional quantity of water wiU be taken up by it, because the 

 first immersion saturated it; in like manner, when air has taken 

 up as much moisture as it can contain, it is said to be in a state of 

 saturation. If when thus saturated a reduction of temperature takes 

 place, the capacity of the air for moisture is diminished, and precipita- 

 tion ensues. "When air, on the contrary, is in an undersaturated, or 

 dry state, it takes up moisture from the substances with which it comes 

 in contact. If moist air is brought into contact with a substance 

 sufficiently colder, a part of the moisture is condensed, and is so con- 

 verted from a state of invisible vapour into water. If, for instance, a 

 cold wine-glass is brought into a warm room, the sides of the glass 

 become covered with dew, which is the water that existed in the air as 

 vapour, and which, condensed on the cold glass, is changed into water. 

 The effect, therefore, of bringing warm moist air into contact with a 

 cold surface is to rob the air of a part of its moisture. Thus, in a cold 

 night, the glass roof of a greenhouse may be seen streaming with 

 water, which runs down and forms "drip," and in this often unsus- 



