14 Things not generally Known. 



and concludes that its weight cannot exceed the 

 of a grain. 



This curious calculation was made by Dr. Thomson, in order 

 to show to what degree Matter could be divided, and still 

 be sensible to the eye. He dissolved a grain of nitrate of lead 

 in 500,000 grains of water, and passed through the solution 

 a current of sulphuretted hydrogen ; when the whole liquid 

 became sensibly discoloured. Now, a grain of water may 

 be regarded as being almost equal to a drop of that liquid, 

 and a drop may be easily spread out so as to cover a square 

 inch of surface. But under an ordinary microscope the mil- 

 lionth of a square inch may be distinguished by the eye. The 

 water, therefore, could be divided into 500,000,000,000 parts. 

 But the lead in a grain of nitrate of lead weighs 0*62 of a 

 grain ; an atom of lead, accordingly, cannot weigh more than 

 sioooooooooo th of a grain ; while the atom of sulphur, 

 which in combination with the lead rendered it visible, could 

 not weigh more than ^orgooooooooo? that is, the two-billionth 

 part of a grain. Professor Low ; Jameson's Journal, No. 106. 



WEIGHT OF AIE. 



Air can be so rarefied that the contents of a cubic foot shall 

 not weigh the tenth part of a grain : if a quantity that would 

 fill a space the hundredth part of an inch in diameter be sepa- 

 rated from the rest, the air will still be found there, and we 

 may reasonably conceive that there may be several particles 



resent, though the weight is less than the seventeen-hun- 

 red-millionth of a grain. 



DURATION OF THE PYRAMID. 



The great reason of the duration of the pyramid above all 

 other forms is, that it is most fitted to resist the force of gra- 

 vitation. Thus the Pyramids of Egypt are the oldest monu- 

 ments in the world. 



INERTIA ILLUSTRATED. 



Many things of common occurrence (says Professor Tyndall) 

 are to be explained by reference to the quality of inactivity. 

 We will here state a few of them. 



When a railway train is moving, if it strike against any ob- 

 stacle which arrests its motion, the passengers are thrown 

 forward in the direction in which the train was proceeding. 

 Such accidents often occur on a small scale, in attaching car- 

 riages at railway stations. The reasofi is, that the passengers 

 share the motion of the train, and, as matter, they tend to 

 persist in motion. When the train is suddenly checked, this 

 tendency exhibits itself by the falling forward referred to. In 



