DEFINITE QUANTITY OF HEAT REQUIRED. 477 



six weeks which it takes four or five months to accomplish in beautiful 

 Italy." 



Attempts have been made to establish the doctrine that every plant 

 requires, from the time of its germination to the close of its Definite quan- 

 organic activity, a definite amount of heat. The following quf r ed by^ ' 

 example, in the case of barley, is furnished by Schleiden. plants. 

 " In Egypt, on the banks of the Nile, barley is sown at the end of No- 

 vember, and harvested at the end of February ; the period of vegetation, 

 therefore, amounts to about 90 days, and the meail temperature of this 

 season is 69 48'. ,In Tuqueres, near to Cumbal, under the equator, the 

 time of sowing in the mountains for barley is about the 1st of June, the 

 time of harvest the middle of November ; the mean temperature of this 

 vegetating season of 168 days is 50 12'. At Santa Fe de Bogota they 

 number 122 days between seed-time and harvest, with a mean tempera- 

 ture of 57 24'. If, now, the number of days is multiplied by the figures 

 of the mean temperature, we obtain 6282 for Egypt, 8433-|^ for Tuque- 

 res, for Santa Fe 6489-|-J ; therefore as nearly the same number as the 

 uncertainty in the estimate of the days, the accurate mean temperature, 

 and the want of knowledge whether or not the same kind of barley is 

 cultivated in all the places, will allow us to expect. Similar results are 

 obtained for wheat, maize, the potato, and other cultivated plants. We 

 may express these results thus : Every cultivated plant requires a cer- 

 tain quantity of heat for its development, but it is the same thing wheth- 

 er this heat is distributed over a shorter or longer space of time, so that 

 certain limits are not exceeded ; for where the mean temperature sinks be- 

 low 36 24', or where it rises above 71 36', barley will no longer ripen. 

 Consequently, to define accurately the conditions of temperature which a 

 plant requires to maintain it in a flourishing condition, we must state 

 within what limits its period of vegetation may vary, and what quantity 

 of heat it requires. This most remarkable circumstance was first ob- 

 served by Boussingault, but, unfortunately, we as yet possess not nearly 

 sufficiently accurate accounts of the conditions of culture in the various 

 regions of the earth to enable us to follow out this ingenious view in all 

 its details." 



Respecting the calculations offered in the preceding paragraph, the re- 

 mark may be made that they contain an element which vi- xhe effect of 

 tiates their correctness, and that, if the proper data were re- the intensity 



, , , . . , . -i -i T -i -i an( i quantity 



sorted to, the general principle intended to be demonstrated O f heat consid- 

 would be far more clearly established. The degrees of the ered - 

 thermometer are not the data required, for that instrument indicates the 

 intensity, but not the quantity of heat. If some form of calorimeter were 

 substituted for it, the result would turn out very differently. As an illus- 

 tration, if a mass of ice of constant surface was exposed to the warmth in 



