138 THE GARDENER. [March 



water (produced by evaporation from water in some of its many forms, 

 and its amount therefore varying with the temperature), and carbon 

 dioxide (carbonic acid) and ammonia, which is composed of nitrogen 

 and hydrogen. The vapour of water is the lightest of all gases, 

 except hydrogen and ammonia, is invisible, is largely absorbed by 

 plants, but on its condensation by cooling is visible as a cloud, mist, 

 dew, rain, snow, or water, and to the latter forms of it we are in- 

 debted for the supply of water to our springs and rivers. It is to 

 the vapour of water that we are indebted for the beautiful appear- 

 ance of the atmosphere, which by preventing the too rapid radiation 

 of heat from the earth, affords as it were clothing and protection 

 to vegetable life. 



A knowledge of the attributes of vapour of water or aqueous vapour 

 will enable the gardener to understand and overcome many difficulties 

 when he works in the plant houses, where he will soon see the 

 effects of aqueous vapour evaporating from the open water tank, or from 

 pools of water purposely thrown down, in its condensation on the 

 cool glass, dripping down, and often spoiling the foliage of plants, 

 especially of orchids j or in a vinery filled with late Grapes, if the 

 lights are left open in dull moist weather, the aqueous vapour of the 

 atmosphere will enter and condense on the cold berries of the Grapes 

 and lay the foundation for many a rotten berry. 



A knowledge of some of the laws of heat w r ill be very serviceable ; 

 without heat the sap cannot rise in the plant cells, nor the seed 

 germinate. To the expansion by heat of all bodies (whether solid, 

 liquid, or gaseous) the gardener is indebted for the thermometer, 

 which, by the rising or expansion of the mercury in the tube, shows 

 on the face of it the degree of heat, whether in the open air, on the 

 surface of the earth, or in the plant house, or even in the hot bed 

 formed of stable-manure (if a thermometer specially constructed for 

 that purpose is plunged into it), a great advantage by-the-by over the 

 old fashioned way of leaving a stick plunged in the hot bed and 

 pulling it out occasionally, and by the personal contact with the hand 

 guessing whether the heat called bottom-heat in the hot bed was suffi- 

 cient. Again, to the laws of the expansion by heat as applied to 

 fluids, the gardener is indebted for the power of heating with the 

 hot-water apparatus, as it is often called, the houses placed under his 

 care ; and to the pressure of the weight of the atmosphere the 

 gardener is indebted for the barometer and the common pump. 



To return to that part of our remarks in which carbon dioxide 

 (carbonic acid) was mentioned as a constituent of our atmosphere. 

 It is the great material with which the plant builds itself; most of the 

 carbon in the plant is formed from carbon dioxide. It forms only a 



