254 PROGRESS IN CHEMISTRY. 



of silver chloride, leaving- sodium nitrate in solution. By the new 

 views, such an equation must be written 



+ — + — + — 



Ag.Aq+NO,.A(i+Na.Aq+Cl. Aq=AgCl+Na.Aq+NO,. Aq. 



The compound, silver chloride, being insoluble in water, is formed 

 by the union of the ions Ag and CI and their consequent discharge, 

 forming an electrically neutral compound; while the sodium ions, 

 charged positivel}^ together with the NO., ions, negatively charged, 

 remain in solution. 



One more application of the principle may ))e given. Many observ- 

 ers, Andrews, Favre, and Silbermann, but especially Julius Thomsen, 

 of Copenhagen, and M. Berthelot, of Paris, have devoted much labor 

 and time to the measurement of the heat evolved during chemical 

 reactions. Now, while very different amounts of heat are evolved 

 when chlorine, bromine, or iodine combine, respectively, with sodium 

 or potassium, the number of heat units evolved on neutralizing sodium 

 or potassium hydroxide with hydrochloric, hydrobromic, hydriodic, or 

 nitric acids is always about 18,500. How can this fact be explained? 

 It finds its explanation as follows: These acids and bases are ionized 

 in solution, as shown in the equation: 



+ — + - + — 



H.Aq+Cl.Aq+Na.Aq+OH.Aq = H.()H+Na.Aq+Cl.Aq. 



Water is the only compound formed, and it is produced by the union 

 of the hydrogen ion originally jjelonging to the acid and the OH, or 

 hydroxyl ion originally belonging to the base. No further change has 

 occurred; hence the uniform evolution of heat by the interaction of 

 equivalent quantities of these acids and bases. 



PEmODIl" AKKANGEMENT OF THE ELEMENTS. 



It now remains to give a short account of the greatest generalization 

 which has as yet been made in chemistry; it has lieen termed the 

 periodic arrangement of the elements. 



In 1864, Newlands, of London, and Lothar Me3^er, late of Tu])ingen, 

 found that bj^ arranging the elements in the order of their atomic 

 weights certain regularities were to be observed between each element, 

 and in general the eighth in succession from it, in the order of their 

 numerical value. Such similar elements formed groups or quantities, 

 while the elements separating them belong to a period, hence the name 

 " periodic arrangement.'' Conunencing with lithium, a light, lustrous 

 metal found as silicate in certain minerals, we have the following series. 



