1859] WRITINGS OF JOSEPH HENRY. 175 



fiuence; for example, the nauseous effluvium which arises in 

 some processes of the arts becomes often insensible to the 

 operator, and the same may be said in regard to the effect of 

 animal effluvia on the inmates of crowded and ill-ventilated 

 houses. The sense of smell, like our moral faculties, thus 

 becomes blunted by misuse or improper association. 



Matter in the aeriform condition is generally transparent, 

 though different gases exhibit occasionally different colors; 

 even the atmosphere possesses this property in a slight degree, 

 as is evident in the fact of the slightly blue appearance of 

 distant objects. 



From all that we have said, it appears that the aerial 

 ocean, like the aqueous one, is a vast reservoir, principally 

 composed of two ingredients of nearly constant proportions, 

 and a number of adventitious materials which in some 

 cases, though in very minute quantities, have a marked in- 

 fluence on animal and vegetable life. There is however 

 another variable ingredient, (previously alluded to in a 

 general way,) which by its production and condensation, is 

 the agent to which nearl}^ all the fitful variations in our 

 atmosphere are to be ascribed. I allude to the aqueous vapor 

 of the atmosphere. But before proceeding to consider this, 

 it will be necessary to treat more fully of some of the prin- 

 ciples of heat and its influence on the climates of the earth. 



Maxima and Minima of Temperature. — A certain degree of 

 heat is necessary to give mobility to the sap of plants, and 

 this differs in each species of plant. Vegetation is acceler- 

 ated and becomes luxuriant, provided it is furnished with a 

 corresponding amount of humidity to compensate for the 

 evaporation as we increase the quantity of heat. It is there- 

 fore important to determine the average amount of heat in 

 different places; but for this certain precautions are indis- 

 pensable. It is not the direct heat of the sun that we at 

 first wish to ascertain, but that of the air. Hence it is neces- 

 sary to suspend the thermometer to a badly-conducting 

 body, and the instrument itself should not have so great a 

 volume as would prevent its readily taking the temperature 

 of the atmosphere. If the bulb is large and the stem small, 



