This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
This is a volatile oil obtained from mentha viri-'dis, a herb cultivated in Europe and the United States. The fresh herb is distilled with water or by means of steam; the yield is about one-quarter to one-half per cent. Oil of spearmint resembles oil of peppermint in color, specific gravity, etc. Its boiling point is said to be at 160° C. (320° F.); after Muspratt at 170° 0.