200 MECHANICS AND USEFUL ARTS. 



1 . Absolute calmness in the atmosphere. 



2. Exposure of the flame to the influence of intense solar light, without 

 heating the surrounding air. 



The first condition was secured by performing all of the experiments in a 

 large lecture room, with all of the doors and windows closed. To secure 

 the second condition, he employed a portion ot the apparatus belonging to 

 a large solar microscope, consising of the reflecting mirror, the condensing 

 lens and tube, together with the mechanical arrangements for adjusting the 

 direction of the light. As the condensing lens was upwards of four inches 

 in diameter, the intensity of the light could be increased nearly tenfold : so 

 that its effects ought to be enormously exaggerated. This arrangement cut 

 off all of the influence of exterior agitations of the atmosphere ; while the 

 concentrated pencil of light, thus thrown on the flame, traversed it, as well 

 as the surrounding air, without imparting a sensible amount of heat to the 

 latter. 



In his experiments, Prof. Le Conte used the best wax-candles. By al- 

 lowing them to burn a sufficient length of time to form a well-defined cup 

 for the melted wax, and carefully turning the wicks so as to render them self- 

 snuffing, the combustion was found to go on with remarkable uniformity in a 

 calm atmosphere. The rate of burning was determined in the following 

 manner : A portion of candle, three or four inches in length, was secured 

 to the bottom of one of the scale pans of a tall balance and ignited ; after 

 allowing it to burn for ten or fifteen minutes, so as to secure a steady flame 

 of constant size, it was nearly balanced by added weights to the opposite 

 scale-pan, allowing a slight preponderance to the candle-pan. In a short time 

 the equilibrium was established by the burning of the candle ; the precise 

 time at which the balance indicated a condition of equilibrium was accwate/y 

 noted. Next, a given weight, (say sixty or one hundred grains,) was with- 

 drawn from the weight-pan, and the time of restoring the equilibrium by the 

 loss of weight in the burning candle, was, in like manner, recorded. In this 

 manner, the rate of combustion was determined by observing the time occu- 

 pied in consuming a given iceight of the burning matter. The arrangements 

 described above, enabled him to perform such experiments alternately in the 

 darkened room, and in the concentrated sunbeam, without moving any por- 

 tion of the apparatus in the room, and under external conditions as nearly 

 identical as could be desired. Many preliminary experinents were made for 

 the purpose of testing the delicacy of the arrangements, which very soon 

 showed that no reliable results could be obtained unless the air was calm, 

 and also, unless the candle was allowed to burn a sufficient length of time to 

 establish regularity in the process of combustion. The days selected for the 

 experiments, were perfectly cloudless. The state of the barometer and ther- 

 mometer were carefully noted. The cone of sunlight was. so direct, that its 

 lower margin illuminated the charred portion of the wick of the candle, 

 while the upper boundary of the pencil traversed the flame near its apex. 



Under these circumstances, Prof. Le Conte found the rate of combustion 

 to be sensibly the same in the dark and in the sunshine ; the slight varia- 

 tions not exceeding the limits of experimeatal inaccuracies. These negative 



