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In addition to the reason why even if a small hole is corroded, it may not be able to blow up the iron city gate, there is another important reason - with the reaction speed of the whole iron city gate and dilute sulfuric acid, it is estimated that it will take several days to corrode before the hole is generated. And for such a long time, the defenders of the city can definitely react accordingly.

Moreover, such a long corrosion time, in addition to the fact that the defenders in the city may not give you time to slowly corrode, it is easy to leak secrets, which is also a very important problem.

In this era, strong acids can corrode steel, and it is not a common chemical knowledge that everyone knows. This is because modern chemistry has not yet emerged, and European chemistry is still in the stage of ignorance and backwardness of alchemists. Even, even if many alchemists can make dilute sulfuric acid, they can't understand the chemical principles.

In the future, Marin still wants to rely on the white iron gate to defend against the enemy's attack, how can he expose the chemical knowledge that dilute sulfuric acid can corrode the big iron gate in public?

In fact, if it is replaced with the extremely domineering effect of the king water, it is very in line with Marin's requirements. The water of the king was splashed on the iron gate, and in a few minutes it corroded a large hole. If the amount is sufficient, it is not impossible to corrode and collapse the entire city gate.

But the problem is that aqua regia is a three-to-one ratio of concentrated hydrochloric acid and concentrated nitric acid. Marin has now mastered the preparation method of concentrated sulfuric acid and concentrated nitric acid, but the preparation method of concentrated hydrochloric acid has not yet been mastered. Although concentrated sulfuric acid and table salt can be heated to obtain hydrogen chloride gas, hydrochloric acid can be obtained by collecting water. However, this can only yield dilute hydrochloric acid. If you want to get concentrated hydrochloric acid, it is not so simple.

Because hydrochloric acid and nitric acid are both volatile acids, unlike sulfuric acid, which can be purified to obtain concentrated sulfuric acid when heated. If dilute hydrochloric acid is heated, hydrogen chloride gas may volatilize before water......

Therefore, concentrated hydrochloric acid is more difficult to produce, and aqua regia is difficult to obtain. Even if Marin can develop aqua regia, it is estimated that it will be a long time. By that time, the daylily would have been cold for a long time......

……

Marin began to think about countermeasures, and after thinking about the problem of easy to flow down after pouring dilute sulfuric acid on the iron door, Marlin got a solution - that is, use a brush to brush the dilute sulfuric acid on the iron door like paint...... In this way, the possibility of going down is reduced......

In order to enhance the viscosity and prevent the dilute sulfuric acid from flowing down quickly, Marin even let people mix some special ingredients into the dilute sulfuric acid, which increases the viscosity of the dilute sulfuric acid and makes it easier to flow down the iron gate. As long as the dilute sulfuric acid adheres to the iron gate for a long time, the reaction time is long enough, it is definitely much better than flowing to the ground quickly......

As for the problem that brushes are susceptible to sulfuric acid corrosion, this one is very simple - prepare more brushes......

However, even if a small hole is corroded in the iron door, the big problem of putting a powder keg in it may not be able to explode, there is still no way to solve it......

……

After trying to pour the wastewater from a solution of paint ingredients mixed with dilute sulfuric acid into a waste ditch, Marin suddenly asked:

"By the way, take a closer look and see where the drain exit of the city of Siena is. A huge city of Siena, there must be a drainage ditch, right? Look for it! ”

After asking this sentence, Marin suddenly remembered that in many movies in later generations, it seems that many spies and assassins sneaked into the city or the mansion through the sewers. Perhaps, this can be done...... The iron city gate is difficult to deal with, is the sewer exit also an iron gate?

……

Marin guessed correctly - the sewer exits in Siena City are indeed iron gates, and they are very thick......

After careful investigation, Marin's special forces fighters discovered that the city of Siena did indeed have a sewer exit, near the northeast gate.

Of course, the smell of the sewer is just too bad. Therefore, this sewer exit is a short distance from the northeast gate, so as not to affect the defenders at the head of the sewer.

However, because the whole city is divided in three places: northwest, due south and northeast. Therefore, this sewer exit is not too far from the northeast gate.

It's just that the architects who designed the city were clever enough to use a small ditch outside the city gate as an outlet for the sewers. Even, the skilled architects, on top of the sewer outlet near the ditch and the northeast gate, added a layer of "cover" (that is, laid a layer of bricks to cover the top) to perfectly cover this sewer exit. If you didn't peel off the bricks, you wouldn't have thought that this was the sewer outlet of the city of Siena.

Marin's special forces fighters also judged that this was the sewer exit of the city of Siena based on the unpleasant smell. After all, the sewer outlet certainly doesn't smell good......

But after peeling off the brick cover, the soldiers of the special forces were also dumbfounded - because, at the exit of the sewer, there is also an iron gate......

The gate is small, less than 1 meter high, and looks more like a big dog hole. However, in order not to affect the outward discharge of sewage in the city, this iron gate is not completely closed, but a large iron gate similar to the steel structure of later generations.

In the middle of the night, Marin personally put on his mask and went to investigate. In the end, Marin came to the conclusion that the steel iron gate at the sewer exit, although extremely strong, was much easier to deal with than the real city gate.

Of course, you can't get rid of this iron dog cave door with great fanfare. This dog cave door, even if this steel door is broken, it will be difficult for Marin's soldiers to enter the city through this big dog hole less than one meter high. Because, this dog hole is too small, if the enemy army in the city finds that someone has broken through from here, as long as a small team of men and horses is dispatched, this dog hole can be easily defended. Because, this hole is too small, and a large group of people can't rush into it at all. Even if you smash open the iron door, you can only climb in one by one. And in the process of climbing in, the other party only needs to send a few people with knives to wait on the inside of the dog hole, and cut off one of the heads, and it will be easy to solve the problem......

……

Therefore, even if Marin knew that the iron gate that destroyed this dog cave could send people into the city, he would not be able to make a big deal of it. Because, once discovered by the enemy, it is very easy for the opponent to defend......

Unless...... Unless you can unknowingly destroy the iron door of the dog hole at the exit of the sewer, and then quietly send someone to drill into the "dog hole", the effect can be achieved. The main thing is to let a few "fierce generals" drill in first, and then, block the enemy troops who come to counterattack, and provide protection for the subsequent entry of men and horses......

It is worth noting that although the defenders of the city sometimes send people to watch the sewer exit, no defenders will guard the edge of this "dog hole......

Why? Because this is a sewer exit, it smells foul. What warrior wants to stay on the edge of this sewer exit at all times?

Therefore, even the soldiers guarding the exit of this sewer are standing at the foot of the city wall more than ten meters away, looking this way from time to time, and no defender is willing to stand on the edge of this stinky ditch and guard it all the time......

If someone dared to violently break the iron gate at the exit of the sewer, the defenders more than ten meters away would definitely notice. Daytime is the easiest, it can be observed with the naked eye, and it can also be heard. And in the middle of the night, just listen to the sound. Because, in the middle of the night, everything is silent, and if you want to destroy the iron gate of the sewer, you will definitely be heard......

In addition, because the sewer exit outside the city is cleverly camouflaged, few people will generally find the sewer exit. Even if you find it, it's useless - there are heavy iron gates blocking it...... If you dare to destroy violently, the guards of a small team of people are not done in vain, just block it and roar at it, you don't want to break through......

……

Of course, the so-called "heavy" iron gate here is only for the dog hole. Compared to the real iron gate, the sewer gate here is really not thick and heavy...... However, under normal circumstances, attempts to break this iron gate go unnoticed...... It's too hard......

But coincidentally, Marin really has a way to destroy this iron gate quietly......

So, Marin called in credible special forces fighters and ordered:

"You, wearing masks, lurk at the sewer gate in the middle of the night, and use a brush to brush this mixture of dilute sulfuric acid and paint on the dog cave gate...... You take turns brushing until the iron door of the dog cave is corroded and broken...... That is, until I can quietly open the iron gate......"

"What about when you open it?"

"Send a few fierce generals to get into that 'dog hole' first, and cover the team to get into it slowly...... In the end, the inside and the outside should join forces to seize the northeast gate......"

"Fierce general?" Suddenly, all of them turned their eyes to Karn on the side......

"What do you want me to do? Do you want me to drill a dog hole? Karn suddenly felt bad......

"Ahem, let's not talk about this, you guys, let's go and destroy the small iron gate of the 'dog hole' first......"

Modified......