TRANSACTIONS OF SECTION B. 607 



III. Energy values are becoming of greater importance in science, and 

 especially iu chemistry. Changes of state of aggregation should always be noted. 

 Therefore I have used a horizontal arrow to indicate a fused or dissolved sub- 

 stance, an upward pointing arrow to denote a gas, and a downward pointing arrow 

 to denote a precipitate or solid. A common equation becomes, therefore, 



AgNO, + IICl = AgCU + HNO,. 



> — > > 



For the volatilisation of lead we would have in its simplest form 



Pb|+heat = Pb 



+ lieat = Pbt. 



Or, if we wish to make it still more definite, as we may to great advantage, use 

 h.l., h.v., and h.s. to represent respectively the heats of liquefaction, vaporisation, 

 and sublimation ; sp.hl, sp.h and sp.h\ to represent the specific heats in the solid, 



— > 

 liquid, and gaseous states, h.diss for heat of dissociation, and h.f. for heat of 

 formation. Then the equation for the raising of lead from 18° C. to 2,000° C. 

 will be 



Pbt at 18° G. + sp.hi x (M.P.-18) therms = Pb| at melting-point 



+ A.Z = PbatM.P. 

 — > 

 + sp.h X (B.P. - M.P. ) = Pb at boiling-point. 



PbatB.P. + /i.w. = PbtatB.P. ~* 



+ sp.h\ X (2,000 - B.P.) = Pbt at 2,000° C. 



All equations are better written for atomic quantities. The above equation 

 does not note any heat of dissociation of the gas, nor the heat used up in expansion 

 against atmospheric pressure. These also may be indicated. 



Practice in writing such equations leads to more thorough appreciation of the 

 conditions in which chemical changes take place. It is remarkable in how few 

 cases a work like Watts's ' Dictionary of Chemistry' gives all the numerical values 

 for the symbols used above. This method of writing equations is easily extended 

 to compounds. 



In the plaster-of-Paris method results are reached so rapidly, and the method 

 of procedure in different cases is so similar, that a rapid form of note-takino- is 

 allowable and necessary. By paragraphing in the following manner the note-books 

 are more easily e.xamined by the teacher, and permit of readier comparison of metal 

 with metal: — 



PbJ, +0.F. =PbO], brownish red when hot, pale brownish yellow when cold 

 fused oxide melts into the tablet. 



+ K.^S = PbSI brownish black, 

 — >• 



+ HCl = insoluble, 

 — >. 



+ HNO3 = soluble, or decomposed. 

 — * 

 Selected equation. PbO| + K.,S = Pb3| + K.,0. 



—-> — -> 



The position of the K.S indicates that the solution is applied to the coatino- of 

 lead oxiile, and the position of HCl that it is applied to the lead sulnhide" so 

 also the HNO3. ^ ' 



