236 THE NUMERICAL RELATION 



different, and may be said to characterize its series. In the first it is nine, in the 

 second eight, in the third six, in the fourth five, in the fifth four, and in the last three. 

 The discovery of this simple numerical relation, which includes all others that have 

 ever been noticed, was the result of a classification of the chemical elements made for 

 the purpose of exhibiting their analogies in the lecture-room. A short notice of this 

 classification will, therefore, make a natural introduction to the subject. 



Every teacher of Chemistry must have felt the want of some system of classification 

 like those which so greatly facilitate the acquisition of the natural-history sciences. 

 In most elementary text-books on Chemistry, the elements are grouped together with 

 little regard to their analogies. Oxygen, Hydrogen, and Nitrogen are usually placed 

 first, and therefore together, although there are hardly to be found three elements more 

 dissimilar ; again. Phosphorus and Sulphur, which are not chemically allied, are fre- 

 quently placed consecutively, while Arsenic, Antimony, and Bismuth, in spite of their 

 close analogies with Phosphorus, are described in a difi"erent part of the book. This 

 confusion, which arises in part from retaining the artificial classification of the elements 

 into metals and metalloids, is a source of great difficulty to the learner, since it obliges 

 him to retain in his memory a large number of apparently disconnected facts. In 

 order to meet this difficulty, a classification of the elements into six groups, difiering 

 but slightly from that given in the table accompanying this memoir, was made. The 

 object of the classification was simply to facilitate the acquisition of Chemistry, by 

 bringing together such elements as were allied in their chemical relations considered 

 collectively. As the classification has been in use for some time in the courses of lec- 

 tures on Chemistry given in Harvard University, I have had an opportunity for ob- 

 serving its value in teaching, and cannot but feel that the object for which it was made 

 has been in a great measure attained. The series which is headed The Six Series 

 will illustrate the advantage gained from the classification in a course of lectures, the 

 elements which compose it being among those especially dwelt upon in lectures to 

 medical students, and, generally, veiy widely separated in a text-book on the science. 

 As Chemistry is usually taught, the properties of the members of this series. Nitro- 

 gen, Phosphorus, Arsenic, and Antimony, as well as the composition and properties 

 of their compounds, make up a large body of isolated facts, which, though without 

 any assistance for his memory, the student is expected to retain. Certainly it cannot 

 be wondered at, that he finds this a difficult task. The difficulty can, however, be in a 

 great measure removed, if, after he has been taught that Nitrogen forms two important 

 acids with Oxygen, NO3 and NO5, that it unites with Sulphur and Chlorine to fonn 

 NS3 and NCI3, and also with three equivalents of Hydrogen to form NH3, he is also 



