INTRODUCTION.

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In attempting to supply these defects,considering the importance of the sub-ject, some apology may be necessary; butin giving the result of facts, which havecome under my knowledge in the courseof professional practice, and also of severalexperiments made with the express view ofobtaining the requisite information; it istrusted, that it will be a sufficient excusefor any errors, when it is considered, thatthe path is almost an unbeaten one, andthat little, except general observations,has hitherto been published.

The greatest care has been used in theprosecution of the different experiments,and the most minute details are given, inorder that the reader may be able to judgeof the credit to which they are entitled;my object has been to furnish practical