CONCLUDING REMARKS. 663 



intermixture of effects. The ancient philosophers, in their search after 

 unity, assumed many things to be simple which we now know are 

 really very complex. For example, they supposed that air was an 

 element, and that its essential properties were few and its condition 

 uniform. We now know that the atmosphere is a mixture containing 

 nitrogen, oxygen, carbonic acid, aqueous vapour, ammonia, and many 

 other vapours and gaseous matters, most of which are themselves com- 

 plex substances ; and that in it there float also liquid and solid particles 

 of various kinds, ferruginous and other dust, organic fragments, ova 

 of plants and of animals. Moreover, it is capable of many different 

 conditions, and susceptible of infinite variations of these ; the known 

 conditions including its temperature, density, humidity, electrical state, 

 etc., while there are doubtless other peculiarities yet to be discovered. 

 The atmosphere is therefore the cause of phenomena in which its 

 numerous different agents may be distinguished and the part due to 

 each assigned, as in the phenomena of animal life, vegetable life, fer- 

 mentation, putrefaction, so-called spontaneous generation, etc. This 

 example will sufficiently illustrate how manifold may be the agencies 

 which work in what simple observation would assume to be a single 

 cause. Men had therefore to learn to discriminate the cause of a 

 given effect from other agencies which might be present, and to trace 

 the effects of a given cause amidst a complication of co-existing effects. 

 The art of experiment, which has been so powerful in physical science, 

 though it has its methods and rules, is not a method of interrogating 

 nature essentially different from that of observation. The distinction 

 has been happily expressed thus : Observation is finding a fact, 

 experiment is making one. In experiment some apparatus or ap- 

 pliances are always necessary, and nearly all observations and facts 

 which are of value in establishing general laws are determinations of 

 the quantitative relations between phenomena. Hence science ad- 

 vances by improvements in apparatus, and especially by increase of 

 accuracy and delicacy in instruments of measurement. It is for this 

 reason that we have in the foregoing pages noticed some of the chief 

 steps in the improvement of instruments for measurement, such as the 

 vernier, the sextant, the micrometer, the pendulum, the chronometer, 

 the torsion balance, the galvanometer, etc. As instruments for the 

 better determination of quantitative relations, we may consider also 

 the invention of new methods in mathematics. Rapid advances in 

 physical science were made when mathematics had provided such in-l 

 struments as algebra, trigonometry, logarithms, the Cartesian geometry, 

 and the infinitesimal calculus. 



In glancing at some of the more general characteristics of the science 

 of the present day, we may consider it first in its practical, and secondly 

 in its speculative aspect. Nothing is more striking than the imme- 

 diate and general application of scientific discoveries which is con- 

 tinually made to the purposes of every-day life. In this respect science 



