212 GENERAL CONDITIONS OP SCIENTIFIC RESEARCH. 



abstruseness of many natural phenomena. The powers of 

 man's mind, when compared with many of the problems 

 presented by nature, appear as nothing to infinity, and ifc 

 consequence of this we are obliged to employ every kind 

 of artificial assistance. But these aids have also only a 

 limited power. Telescopes will not enable us to see an 

 infinite distance, nor microscopes to see atoms or even mole- 

 cules. Spectroscopes, polariscopes, and photometers have 

 also a limit of action, and for the aid of the senses of smell 

 and taste we have no appliances. With the assistance of 

 presses we can only produce a finite degree of pressure ; 

 and with the aid of air-pumps we can only rarefy gases to 

 a limited degree. By means of a balance a difference of 

 weight of 1 in 10 millions can hardly be detected, and a 

 smaller quantity than one-hundredth of a milligram cannot 

 at present be weighed. The highest amounts of power 

 and accuracy yet attained are as follows : Mousson esti- 

 mated that he had subjected water to a pressure of 1,300 

 atmospheres, or 8 J tons per square inch ; Whitworth has 

 measured a difference of one-millionth of an inch in 

 length ; and Joule has detected a difference in tempera- 

 ture of one 8,800th of a centigrade degree. By means of 

 a mirror and electric spark Wheatstone measured one 

 72,000th of a second of time. By means of his chronoscope 

 Noble has detected a period of time as small as 1 millionth 

 of a second ; and with the aid of the pendulum Airy ob- 

 served a difference of time of 1 in 10 millions, or 2-1 

 seconds per day. By means of the microscope we can see 

 an object a 50,000th to 100,000th part of an inch in 

 length, but fail to see one of much smaller dimensions. 

 By the aid of the spectroscope, Bunsen detected as small 

 a quantity as one 180-millionth of a grain of sodium. 

 Kessler's reagent renders manifest 1 part of ammonia in 

 more than 100 million parts of water; and the sense of 



