io2 THE POPULAR SCIENCE MONTHLY, 



of a known law, acting perhaps in a peculiar and even unprecedented 

 manner. In such cases the phenomena cease to be residual phenomena. 



But, on the other hand, some of the observed phenomena may resist 

 every attempt made to explain them ; they may refuse to retire from 

 the list of established facts, and at the same time refuse to find their 

 full explanation in terms of any well-established law. But, while so 

 doing, these phenomena may also not be opposed to the law ; they may 

 not be contradictory to, but simply not wholly explainable by, any 

 known law 'of nature. 



Instances of the valuable results which have been obtained by the 

 exact investigation of residual phenomena are numerous in every branch 

 of natural science. One of the most striking is furnished by Newton's 

 investigation of the atmospheric velocity of sound. 



Newton showed that the velocity of sound in air might be calculated 

 from certain theoretical considerations; a rough measurement of the 

 actual velocity gave him a number differing very considerably from that 

 which his theory required. Later and more exact experiments failed 

 to explain the discrepancy, but in 1816 Laplace gave an explanation of 

 the seemingly exceptional phenomenon, which not only sustained the 

 theory of Newton, but also paved the way to the modern doctrine of 

 the equivalency of heat and mechanical work. In the residual phenom- 

 enon which was left unexplained by Newton lay the germ of one of the 

 greatest advances made by science in recent years. 



Another striking instance of the value of residual phenomena is to 

 be found in the history of chemical science. 



From his experiments upon combustion, Lavoisier concluded that 

 the peculiar properties of acids are due to the presence of the element 

 oxygen in these bodies. But an undoubtedly acid substance was known 

 (muriatic acid) from which no oxygen could be obtained. Here was a 

 residual phenomenon a phenomenon not absolutely contradictory of 

 the law, that that group of bodies called acids is characterized by the 

 presence of oxygen, but certainly a phenomenon demanding accurate 

 investigation. Closer examination might have shown that the acid sup- 

 posed to contain no oxygen was not really free from that element, or it 

 might have led to the adoption of a higher generalization concerning 

 the nature of the group " acids," or, lastly, it might have necessitated 

 an entire alteration in the terms of Lavoisier's so-called law. 



Chemists, however, for many years contented themselves with as- 

 serting that, as Lavoisier had pronounced oxygen to be the acidifying 

 principle, and as muriatic acid was undoubtedly a true acid, this body 

 must contain oxygen. But Sir Humphry Davy showed that an accu- 

 rate examination of the residual phenomenon presented by muriatic 

 acid led to a more extended and more exact knowledge of the nature 

 of acids, and necessitated a change in the prevalent views concerning 

 these bodies. The views of Lavoisier were found to express a truth, 

 but not the whole truth ; fresh incitement was given to research, and 



