February 7th was a rare day in which Darwin did not enter anything in his diary. It seems that this became less common as the voyage continued – possibly as he became more disciplined about keeping a daily record. Like many days, I suspect he was analyzing the many samples that he collected on the island.
Analysis of samples in the field in the early 1800’s was conducted in a manner that was not that different how geologists analyze samples today, and Darwin had a full regiment of geology equipment with him on the trip.
First and foremost, he couldn’t call himself a geologist without a rock hammer for breaking through the weathered surface of a rock to see its true nature. I was thrilled to find that Darwin’s rock hammer, the very one that traveled with him on the Beagle, was in the Sedgwick Museum of Earth Science. Someday I have to see this in person!
Though I have to say I am amazed it still exists – I think my first rock hammer lasted about 2 years before I lost it in the field. On a college trip. If Darwin left his hammer in Cape Verde or Terra del Fuego, there would have been no going back to find it later!
In the field Darwin would have also used his “geological compass” or clinometer (geologists often refer to this as a “Brunton” or “Brunton Compass” today). The clinometer allows one to accurately describe the orientation (strike and dip) of layers of rock in the field. It can also serve as a survey tool for triangulating locations and heights.
Darwin’s Clinometers from serc.carlton.edu:
Darwin apparently struggled making measurements with a clinometer. Though a few months before the voyage he wrote to his mentor J.S. Henslow to say how he had been practicing with his new toy:
“I should have written to you sometime ago, only I was determined to wait for the Clinometer: & I am very glad to say I think it will answer admirably: I put all the tables in my bedroom, at every conceivable angle & direction I will venture to say I have measured them as accurately as any Geologist going could do.” (Correspondence with J.S. Henslow on July 12, 1831)
He seems to spend some time in the early parts of the voyage continuing to make sure he was using it correctly. Apparently, the accurate measurements of the Baobab tree in Cape Verde where made by Fitzroy, as Darwin may have still been struggling to master the technique. There is some solace in the fact that measuring strike and dip with a Brunton Compass is one of the more difficult concepts for modern geology students to master – they were not alone!
Back on board the ship (or in the tent) Darwin could example the samples he collected using several tools designed to help identify minerals. These included a goniometer and a set of blowpipes. Every mineral has a unique chemical composition and chemical structure (arrangement of atoms). This “crystalline structure” is reflected in the outward crystal shape of a mineral. A goniometer allows the mineralogists to accurately measure the crystal shape (specifically the angle between the flat planes of the mineral surface). This measurement can aid in mineral identification.
Contact goniometer from the Virtual Museum of the History of Mineralogy:
A blowpipe was a tool used to measure the other unique characteristic of a mineral – its chemical composition. The user would place a chip of the mineral in a flame and could blow a concentrated stream of oxygen down the pipe to “fan the flames”. This allowed the temperature of the flame to reach upwards of 1500°C. By “burning” a specimen at these temperatures, observations of the color of the flame or the smell produced could help identify the chemical composition, and thereby help ID the mineral itself.
A blowpipe set in its case and in use from Virtual Museum of the History of Mineralogy:
In Geological observations on the volcanic islands visited during the voyage of H.M.S. Beagle, Darwin refers to measurements made with the blowpipe frequently, including this one from Cape Verde:
“This earthy matter is of a pale yellowish-brown colour, and appears to be a mixture of carbonate of lime with iron; it effervesces with acids, is infusible, but blackens under the blow-pipe, and becomes magnetic.”
Notice Darwin also refers to the geologists trusty “acid test” – testing a mineral composition based on whether or not it reacts with acid. This makes carbonate minerals easy to identify.
Finally, being able to look at rocks (and biological samples) under magnification allowed for detailed descriptions and was often helpful for identification. Therefore, no traveling naturalist (Darwin included) would be without a microscope. Darwin’s would have looked much like this one, from the Whipple Museum of the History of Science:
Of course, all the tools in the world don’t help you if you don’t know how to make careful observations of the world and record them in a manner that makes sense to other scientists. This was Darwin’s true strength and I’ll come back to that in the next couple of days.