Prof. Oersted on the Compression of Gases. 105 



It is very difficult in these experiments to determine with 

 accuracy the volume of the inclosed column of air, it being 

 limited below by a curved plane, the form of which varies ac- 

 cording to the friction between the mercury and the glass. 

 We endeavoured, however, to divide this curve by the eye, 

 into two equal portions ; nevertheless it is proved by the re- 

 sult that we calculated the inclosed air too low. Without this 

 error, the differences would have been smaller, and the num- 

 bers sometimes higher, sometimes lower. But still the dif- 

 ferences are smaller than could have been expected in experi- 

 ments which preclude the use of the vernier. 



In the last experiments, for instance, the height of the ob- 

 served column of air amounted to 56'k millimetres; according 

 to the Mariottic law, it should have been 56*287 ; the whole 

 difference, therefore, is no more then 0*113 millimetres, an 

 error which in such experiments is quite inevitable. In the 

 preceding one, the height of the observed column was =63*17 

 millimetres, which, according to the Mariottic law, should 

 have been =63*99. This difference, the greatest we have had, 

 amounts to 0*82 millimetre ; but being between two observa- 

 tions which offer but a very small deviation, it will not militate 

 against the general law. 



•In order to investigate the compression of the air by greater 

 powers, we made use of the air-gun : the King, whose en- 

 lightened generosity has already done so much for the ad- 

 vancement of science, having placed all the requisite appara- 

 tus at our disposal. Every one knows that in this kind of 

 weapon, it is the but-end which is the reservoir of the com- 

 pressed air. This part, therefore, is always made very strong. 

 We first ascertained the inner space of one by weighing it, 

 first when empty ; and again when filled with water. By this 

 means, the quantity of air that might be contained in it, could 

 be easily ascertained. The one which we used most frequently, 

 could receive 0*891 grammes of air at a height of 0*76 metres 

 of the mercury of the barometer. We were also enabled to 

 determine by weight the degree of density we obtained in our 

 experiments ; a method which proved sufficiently accurate, as 

 the scales we made use of were affected by one centigramme. 

 We succeeded in forcing into one but-end, 101*2 grammes of 

 air, a quantity answering to the pressure of 1 10*5 atmospheres. 

 We also took into consideration the expansion caused by the 

 pressure from within on the reservoir, and determined it by 

 weighing in water the but-end empty, and filled with air. In 

 calculating, we assumed, that the different degrees of this ex- 

 Vol. 68. No. 340. Aug. 1826. O pansion 



