has arrived amounts to certainty, or only to high probability, and will separate the doubts and difficulties with which the question has been encompassed by the sophistry of counsel, from those that belong intrinsically to the subject, and are inseparable from it. And it may be proper on this occasion to observe that the medical practitioner is not to withhold an opinion because it may be involved in doubt; he is to furnish the best evidence which the nature of the case will allow, and when he duly performs this task, he may feel proud in the consciousness that he occupies an important station in the administration of justice; and that he conscientiously discharges a duty, without the due performance of which, the laws of his country would be inoperative. ARSON. The charge of _Arson_[602] may occasionally become the subject of scientific research, and the accused individual receive an honourable acquittal at the hands of the chemical philosopher; by whose interposition, the conflagration, unjustly imputed to malice, may be proved to have originated from a spontaneous process of decomposition. _Spontaneous Combustion_ may be defined, _an inflammation occasioned by the re-action of different bodies upon each other, at the ordinary heat of the atmosphere, without the contact or approach of any other body previously raised to a high temperature_. This definition necessarily excludes that class of substances which evolve gaseous matter of a highly inflammable nature, but which requires the approach of an ignited body to kindle it. The subject of _spontaneous combustion_ has attracted the attention of many very eminent chemists, and an extensive series of experiments has been instituted in several different countries for its complete investigation, the results of which have thrown considerable light upon the causes which operate in the production of the phenomenon, as well as upon the nature of the substances most liable to such an accension, and the particular circumstances which are essential to its occurrence. The following may be considered as the principal sources from which it may originate, viz. I. FRICTION. II. FERMENTATION OF VEGETABLE AND ANIMAL SUBSTANCES, as that of _hay_, _oatmeal_, _roasted bran_, _coffee_, _&c._ _rags in paper-mills_, _&c._ III. CHEMICAL ACTION. _Accension of oils, by various animal, vegetable, and mineral substances; accension of vegetable matter by concentrated acids; ignition of lime by the affusion of water; ignition of pyrites._ We shall proceed to consider these subjects more in detail.