332 DIRECT PROPERTIES OF MOLECULES 118 



These numbers have for unit of length the millionth 

 part of a millimetre. 



119. Section and Volume of Molecules 



Although we may not pretend to see an exact evaluation 

 of the size of the molecular diameter in the value 0-2, which 

 we have assumed as a mean, yet it seems justifiable to 

 suppose that this number may serve as an approximately 

 correct estimate. It is therefore not lost trouble, and it is 

 more than a play with figures, if we calculate the sectional 

 area and the volume of a molecule from this estimate of its 

 diameter. We shall be conscious that the calculation can 

 give us only approximate values, since we must once more 

 introduce the assumption of a spherical figure, which is not 

 strictly correct. 



If, then, we put s- = 2 x 10~ 8 cm. as the average 

 diameter of a gaseous molecule, its sectional area will be 

 i-TTs 2 = 3 x 10~ 16 sq. cm., and its volume ^Trs 3 = 4 x 10~ 24 ccm. 

 Keferred to the millionth part of a millimetre as unit of 

 length, these numbers are 0'2, 0'03, 0-004 respectively, the 

 units of area and volume then being the face and volume of 

 a cube of which the edge is a millionth of a millimetre. 



12O. Number and Distance apart of Molecules 



Now that we have attained to a knowledge of the size 

 of the molecules, there opens out the possibility of taking 

 a further step towards the knowledge and measurement of 

 an invisible world by determining first the value of N, or 

 the number of molecules contained in unit volume. This is 

 at once obtained if we compare the value of the sectional 

 area just computed with the sum of the sections Q, 

 discussed in 109, which we likewise know in absolute 

 measure, having calculated it numerically in 110. By this 

 magnitude Q we understand the area covered when we range 

 close together on a plane all the molecules contained in one 

 cubic centimetre of a gas under atmospheric pressure. Since 

 we now know, at least approximately, the size of the 



