XVI

P R

E F A C E.

on certain occasions, even to avoid errors: indeed in d!-rect experimental enquiries, the effect of the operation issometimes so much influenced uy circumstances whichare apt to pass unheeded, that it is not to be wondered iferrors are to be found in the writings even of the most ac-curate experimental chemists.

The history of each subject I have made as complete,as my present means of information, and the experimentsI have hitherto made, will permit; but much remains stillundetermined and unexamined : if the work goes on, andthe author should be able to execute it to the utmost ofhis hopes, the publick is not to expect that any article willbe perfect: researches in chemical knowledge we can ne-ver hope to make complete, every new acquisition shewingnew paths for our further progress in a province of un-bounded extent. Such material deficiences as occurredupon a review of the volume, and some mistakes which Ihad been led into either by my own inadvertence or theauthority of others, are taken notice of in the appendix.It is hoped that the fame regard for the publick, whichinfluences the author, will prevail on those who may dis-cover any other mistakes of moment, to give him an ac-count of them, that they may be rectified in some futurepublication. It is hoped also, that from the communica-tion of friends and artists, much of what is wanting inone publication may be supplied in the next.

One of the principal obstacles to the prosecution ofchemical enquiries has hitherto been the want of a proper

apparatus.