Zwaardemaker's nine odour classes

Zwaardemaker kept Linnaeus's frame of seven classes and added two: the ethereal class (odores aetherei) from Lorry, and the empyreumatic class (odores empyreumatici) from Haller. He explained that the old seven classes were drawn up for the remedies of Linnaeus's day; modern chemistry needed room for fruit esters and for the smell of smoke, tar and benzene.

The table applies only to substances that act purely on smell. He set aside two parallel series: odorants accompanied by a pungent feeling, in Fröhlich's term, and odorants that are also tasted. Each of these, he wrote, would need its own system.

What is new is the column of examples. Rimmel took one material as the type of each class; Zwaardemaker, where he could, took a compound of known structure. The aromatic class is split into five subclasses, each with a type compound: borneol for the camphor group, eugenol for the spicy group, anethole, menthol and thymol for the anise and lavender group, geraniol and citral for the lemon and rose group, benzaldehyde for the almond group.

The examples column is taken from the summary at the end of chapter XIII. The library column links only natural materials that appear in his list.

Zwaardemaker cites Rimmel many times. The ethereal class starts from Rimmel's fruit series, with pear as the type. Chemistry then showed that pear, apple and pineapple smells all come from esters, so a group perfumers had formed by perception matched a chemical group. The aromatic class combines nine of Rimmel's series, the fragrant class five others.

He also recorded where things did not match. Some oils, such as that of parsnip fruits (Pastinaca), contain esters yet had been placed elsewhere by odour. His remark: similar odours usually have analogous composition, but substances of analogous composition do not necessarily smell alike. The question is still open; a 2025 review in Chemical Senses concludes that no universal relation between odour quality and physical or chemical parameters has been found.

At the end of the chapter he tried reducing the nine classes to two large groups. The first four (ethereal, aromatic, balsamic, amber and musk) are food odours and provoke no reflex; the last five are odours of decomposition and do. He proposed these two groups as a provisional frame for the pungent and the tasted series as well.

The instrument that made Zwaardemaker's name predates the book by seven years. He first described it in 1888, in the jubilee volume for the ophthalmologist Donders, then in the Berliner klinische Wochenschrift that year and in The Lancet in 1889. He called it the Riechmesser, or olfactometer.

The design is simple. A curved glass tube, one end placed in the front half of a nostril. Over it slides a cylinder, 10 cm long, made of or coated with the odorous material. Pulling the cylinder out beyond the inner end exposes more odorous surface to the air drawn in, and the stimulus grows; the length can be read off the tube. A small wooden screen closes the other nostril. The strength of the stimulus can thus be written down as a number and repeated.

From this he defined the olfactie: the stimulus at the threshold of a person with normal smell, used as a unit to compare acuity between people. Chapter X of the book also studies compensation: two odours given to the two nostrils, or mixed in suitable proportion, can weaken each other. Such pairs were later called Zwaardemaker pairs.