306 CHEMICAL COMBLNATIOXS 



and will also suffice to show the absolute necessity of maintain- 

 ing an adequate supply of aqueous vapor in the atmosphere of 

 our hot-houses, as well as the imperative necessity of studying 

 and making ourselves acquainted with the nature and qualities 

 of the atmospheric elements. Science has already done much, 

 and is still doing more, for the art of horticulture. We have 

 the thermometer, by which we can deal out heat and cold by 

 the measure. We have the barometer, by which we can ascer- 

 tain to a decimal the weight or density of the air. We have, 

 also, the hygrometer, by which we can tell the precise amount 

 of its contained moisture, — although this latter instrument is 

 but little used in practical horticulture, — and we hope the time 

 is not distant when it will find a place side by side with the 

 thermometer in our hot-houses, to which it does not yield one 

 iota of importance, of interest, or of utility. When shall we have 

 an instrument, equally simple and efficient as these, with which 

 we may ascertain the proportions of its gaseous elements, so 

 that we can regulate the constituents of an atmospheric volume 

 as easily as we can do its heat and moisture ? Such an instru- 

 ment is much wanted by exotic horticulturists, and we trust 

 something of the kind will be yet brought into use. Such an 

 instrument could be applied to excellent purpose, and would be 

 an incalculable boon conferred on gardening, — one almost un- 

 equalled in importance at the present day, and would be of 

 immense utility in all the higher and more difficult branches 

 of exotic horticulture. 



7. There is, probably, no individual branch of natural science 

 so useful in itself to the practical gardener as a knowledge of 

 the various atmospheric phenomena which occur in hot-houses, 

 as well as out of doors; and without we study the one, we can 

 have but little knowledge of the agencies which regulate the 

 other. That a practical foreknowledge or intuitive perception 

 of the ordinary changes of the atmosphere is an acquirement 

 which may certainly be obtained, to a very considerable extent, 

 without the aid of science, is beyond a doubt. We find that the 

 untutored savage, taught only by his own observation, or instinct- 

 ively, is regulated in his movements by an unerring perception 



